I worked in the propane business for about 14 years, and it was my family business since birth, so hopefully I know a little on the subject.
Construction sites and the likes fall under an entirely different set of rules from homes or rvs etc., and can get away with quite a bit of things you would not normally be able to under the regulations as "temporary heating" situations. That tank with the heater mounted to it is perfectly acceptable in a situation like you describe, but you would not want that in a living area, and definitely not where you are going to sleep. First off, even a tiny leak in an enclosed area can build up to the point where the whole thing can blow up when it finds a source of ignition. And second, that type of portable infrared heater can get knocked over, or something can brush up against it, and create a fire hazard. Plus you've got 5 gallons of very flammable liquid under pressure inside the camper with you.
You definitely need to have the tank(s) mounted outside the living area. Either entirely outside, or in a vented compartment with an airtight seal to the interior. So if you are building an enclosed area for the tanks, it would need to be accessed from the outside only and not the inside, and have adequate ventilation (like a screened area, or louvered vent) in the floor or bottom of a sidewall as propane is heavier than air and will sink down if you have a leak. That way if a small leak were to develop on the tank itself, or a connection, or a regulator were to fail, the gas would safely vent to the outside instead of building up inside the rv where you are sleeping. Also be sure to seal any openings or joints into the interior of the unit so gas cannot seep from the compartment to the interior in case of a leak. Ran D. St. Clair has some good pics of the vented compartment he built in his stealth camper build thread. Also be sure the tanks are mounted away from any heat source, the hotter the tank, the higher the pressure. And there is a safety pressure relief which will vent gas if the tank gets too high a pressure, which you don't want to happen. So keep them away from exhaust from the engine, generator, or appliances.
The regulations for camper on tank size depend on the type of tank. The permanently frame mounted ASME tanks have no max size limit at all, but those are fairly expensive. The portable DOT cylinder (like bbq tanks) are limited to a maximum size or 35# per cylinder, a maximum of 3 cylinders, and a maximum of 90# total capacity. In the real world there are 20#, 30# and 40# cylinders, so you could use 3 30# tanks to get to the max. I cheated a little on mine and mounted 2 40# tanks outside under the gooseneck area. I only had room for 2 tanks, but had plenty of height clearance so I opted for the taller tanks. Not quite the correct size, but hey I'm a professional right? lol. I really don't think the size or number of tanks is real important issue safety wise, as long as they are securely mounted, and vented.
A few other things to keep in mind: All gas lines other than the connection from the tank to the regulator need to be metallic. The lines from the regulator to the appliances can be threaded schedule 40 black pipe, or single flared soft copper refrigeration tubing. No rubber hoses (can rub and rupture), no aluminum tubing (susceptible to cracking under vibration), no compression fittings (can crack and leak), no solder joints. If you use soft copper, you can run that directly to each appliance with a shut off valve for each appliance. If you use schedule 40, you will need either a stainless flex connector, or a soft copper connector to the appliance for vibration. Be sure to pressure test the system for leaks before use.
Be sure to use a propane detector installed inside the unit, as well as a carbon monoxide (CO) detector. These are fairly inexpensive and available at most of the rv surplus outlets and on ebay. The gas detector will sound if you have a leak, and the CO detector if you have a carbon monoxide buildup, which can kill you. CO buildup is fairly easy to do in an small enclosed area in you aren't using the proper type of appliances or venting.
Any heater you use should preferably designed specifically for rv use, and if not it at least needs to be designed and approved for residential use in sleeping areas. That sleeping area designation is very important as you have to remember that any gas burning appliance is burning up the same oxygen that you are breathing. A heater designed for sleeping areas has an oxygen depletion sensor built in which will shut the unit down if it uses up too much air. That way you will wake up in the morning. Some wall type heaters are designed for areas like garages or recreation rooms and do not have that feature. The portable tank mounted heaters definitely do not.
I used an Oylmpian Wave Catalytic wall heater in mine:
Olympian Wave Catalytic Safety Heaters - Product - Camping World They are a little pricey, but a nice self contained easy to install unit with all the correct safety features. My living quarters are the front 20' of a gooseneck race car trailer with no significant insulation at all. I used the Wave 6 only because I did not have the room for the Wave 8 on the wall where I wanted to put it. It has 3 settings up to a max 6000btu on high, which does not sound like much, but it really does the job. The only time it is ever on high is to warm up and take the chill off, after that it stays on low (about 3000btu). On a night in the 40's or 50's I leave the roof vent open to let a little heat out or it actually gets to hot inside. We've never used it below the 40's, but I think it would keep up just fine into the 30's. The nice thing for a home built camper, is the unit is completely self contained, just bolt it to the wall and hook up the gas, and operates entirely on propane alone. No 110v, no 12v, so if you are dry camping it will run without having to worry about a thermostat, or a fan to run down the batteries. We are in situations often where we can't plug in, and there are "quiet hours" where we can't run the generator at night, so that unit has worked out very nicely.
Sizing your tanks: Propane has 21600 btu per pound. So for example my 6000 btu heater will burn up a pound of propane in 3.6 hours, so a 20# tank will last 72 hours on high. On low it will burn a pound in 7.2 hours, for 144 hours on a 20# tank. You can figure that usage out for any given heater and make sure you do the math and that you will have enough propane on board or last as long as you will need to. It is far cheaper in the long run to have more capacity on hand so you can fill your tanks at a reputable propane dealer at your convenience, instead of those ridiculously expensive exchange tanks (which they only put 15# in these days) when you run out at inconvenient times and need it in a hurry.
Just remember that propane is very very safe in if installed and used correctly, and very very dangerous if you do not do it correctly.
Well, that is probably way more than you wanted to know, but I figured I'd cover the whole subject in one shot. Good luck!