I am just finishing up an 8ft camper I custom made for myself, all plywood or osb, no tin. Tin sweats and causes rot and mildew so thats the reasoning.
I sealed the unit with liquid rubber, the black stuff is thicker and I used it on the roof and about 6" down each side.
Then I used the white stuff on the walls, its thinner so I thought it would be fine on the walls.
Then to reflect the heat on the black roof I went over the roof with the white and that worked good so....several coats on all surfaces cost me about $250-$300.
No leaks, and its good for 5 years and can be touched up easy and cheap.
Another option to think about....re-plywood then get a duradeck installer to put duradeck over it, its pricey but I checked into it and its an option especially if a pro puts it on and folds the corners properly. If you don't care about color then you may find an installer who has a part roll and you can get that at a discount.
One more option I checked into and almost did...put a torch on membrane over the plywood, get a pro to put that on too, its abit heavy and pricey too. If you know someone in the business you may get the material cheap, like I got a part roll of the membrane from a torch on buddy for free, and he would of torched it on for me for a bottle on a saturday.
The membrane is the kind they put on over a deck before pouring concrete over it, its about 3/16" thick or so.
Hope this gives you some idea's.