We've down a half a dozen different methods of slideouts. We have used several off the shelf brands and a few variations of our own. The first thing I will say is that hydraulics should be out unless you personally have a lot of experience with them. Unless you're using a secondary mechanical alignment system, you will need at least two rams to try to get it in and out straight every time. It takes a lot of effort and valves to make sure that the two rams are working together perfectly. Once you pick up the power pack, pistons, valves and hoses you're gonna be into it for a minimum of $1000. Then you got to worry about support arms and bearings. Further to that there is the inevitbale leaks and seapage.
If you're just running a small (14 x 3') slideout go get the HWH system. It comes with everything you want and need and tells you how to put it together. If you breakdown on the road pats should be close by at the local RV dealer.
If you really want do your own The best way to go would be a bearing guided arm system. Weld some teeth to the arms and power by a shaft with a couple of pinions on it. A strong 12VDC gearmotor is sometimes hard to come by with the right final drive speed so either use an HWH or Atwood motor designed for the job or get a winch motor and adapt it. Winch motors are louder but they take a beating and have a lot of torque to get the job done. Many of the other 12VDC gearmotors available have really weak reduction systems and sometimes don't handle the jarring of starting and stopping that well.
We build a lunch trailer that has opposing slideouts 40' long with 8' of extension. The slideouts actually extend a full 8' first and then lower 4 to 8 inches which gives you a mobile lunch room 24' x 40' with a completely flat floor. Our prototype used a hydraulic pistons to raise/lower and hydraulic orbit motors to turn the extension drive shaft. It was a nightmare at the end of the day and now we strictly use 1HP 120VAC drive motors for extension and Joyce Jax screw jack system for raising and lowering. The slideouts are driven by 3 gearracks mounted to the bottom of the slideout. 2 More gearracks are mounted on each end and this is all driven by connected shafts. This side racks hold the slideout from tipping as it goes in and out and we use no other arms to hold the slideout up. It's a great system which allows us to build gigantic slideouts without massive supports. Since the gear rack is always in contact with the slideout the slideouts interact with the structure of the trailer and actually help to eliminate as much twist and sag in the body. We use this system on all our slideouts these days regardless of size, but it does carry hefty price tag.
I'm sorry if I rambled off there, I was just trying to qualify my opinion on the HWH system. Go get an off the shelf system from HWH or Liftco, it'll save a lot of headache.
