And d.o.t. rules don't take effect until you hit the magic 10k lbs gross combined weight. You could put whatever advertising you want all over your F350 and drive down the road all you want with no dot oversight. What is the gvwr on a F350? about 9600 lbs isn't it? Now, hook up a Home Depot folding trailer to it rated at 1000 lbs. max. You don't even have to put anything on the trailer (or in the bed of the truck for that matter) but now you've got over 10k lbs. gcwr. You got anything commercial going on, boom goes the dynamite. What if you have a Ranger p/u? what are they rated max? 6k lbs? Then hook up a nice flatbed trailer rated for 3500 lbs. You can put "Acme Muffler Shop" signs all over it and haul mufflers to car shows and sell them and anything you want. And there is no dot involved.
Now, you've got a truck conversion rated at 45k lbs. gvwr. Pulling a enclosed stacker trailer rated at 25k lbs. with your personal vehicle inside, and a boat. Absolutely nothing commercial about any of it. You don't even go to a craft show and sell a crochet trivit. You don't have any dot oversight. You're good to go.
But, here comes that "gray" line. You have a car you take to the race track. And the "Acme Muffler Company" gives you free muffler work if you'll put a sticker on your car (doesn't even have to be on the outside of the trailer, it's on the car inside the trailer). You run the car around the track for practice day. No racing, no timing, no prizes, no placing. Just practice. But, a DOT guy is watching you. And your a advertisement for Acme Muffler. So, you're operating in "furtherance of a commercial venture." DOT watches you put that car back in the trailer and drive out. Boom goes the dynamite. Busted. Unless you've got all the dot requirements followed that is. Which is highly unlikely. For one thing you can't have dot numbers on an rv. You're going to need weighted or apportioned plates. It goes on and on and on.
I've got a buddy that is a commercial vehicle enforcement officer. He told me to take the "Recreational Vehicle, not for hire" stickers off my doors because when they see those they figure you are for sure a commercial veh. I've left mine on because they were there when I bought it and I'm guessing the paint might have faded and it'll show that they were pulled off anyway. I am in no way commercial either, so I welcome the chance to be pulled over and prove such to anybody. I've driven almost 40k miles on mine, never been stopped or even attracted any police or dot attention as far as I know.