First of all, my tow vehicle is a 1987 1 ton Toyota truck, with a 2 inch lift, an 31x10.50/15's. So weight isn't even something I'm considering. My design ir almost a plain box. 7.5 ft long and 4.5 feet wide. There won't be a galley as I'm used to either cooking on a stump or my tailgate. Food storage will be the bed of the truck. The rear of the trailer will tilt slightly towards the front at the top. The roof will have a very slight angle so water will run off easily while it's stored at my parents.
The sides will be flat, with doors on each side. The front will be verticle and mostly flat. From the top edge to about 2 feet down will stick out around a foot, to make a shelf above the headboard and be where a window will go. Both doors will be home made. For the finish, I'm thinking just paint after sealing it.
Ok. So now you know what the shell will look like. Sorta.
First question is about the plywood. I have seen pics of plywood used by people here, but I can get this stuff cheaper, $12 for a 4x8 sheet. Is this something that can be used?

I really don't see since it's ratd for outdoor use and for flooring. So it's rather water resistant and has extreme strength. I know that from experience because of my shop and bench. Part of the roof blew off, so I had a lot of water in it. Bench was right under the hole. The bench is made of the plywood, well the top, shelves, and drawers. The top had standing water, shelves had standing water, and a few drawers had water in them. All that for nearly a week, and none of the wood has been treated since I purchased it. So yay or nay?
Next is insulation. I plan on insulating the walls and ceiling. For studs, I will be using 2x2s cut from 16 ft 2x4s. I know 2 inches isn't much clearance, but would fiberglass be ok to use? The foam insulation isn't cheap here, but I can get fiberglass a heck of a lot cheaper. I don't plan on camping in 0 degree weather, so a bit thinner than usual would be ok here. Plus, it doesn't get old enough when I would camp that a 12v heater wouldn't make up the difference. So yay or nay?
Lastly is the finish. What is the most bare bones, cheap as dirt finish that can be done? I think I read something about poly thinned out and put on the wood and soaked in, with regular house paint on top. A few Google searches shoed that would be very very cheap, but opinions are very welcome. So does that sound ok?
Do keep in mind that I am self/unemployed. I restore antique radios as a business but it has slowed to a halt. So my budget is around $300, which is a bit low I know. But I'm not making something that looks like a million bucks, just nice.
BTW, here's my trailer. Ignore the weeds growing around it and the junk in it. Going t drag it to my shop tomorrow.