Craigslist or swap meets for the toolbox. I'm guessing you're looking for the truck bed style. I was at a car show/swap meet couple weekends ago and there were different truck bed tool boxes everywhere for sale. Also, I saw a ton of tires there for cheap. There was a few vendors that had tractor trailers full of tires. Junk yards also have tires for as little as 10 bucks each.
For the table you could basically recycle trash and and some scrap hinges you got laying around, Or for a lot nicer table, you can do what MDB did with the modular shelving on his tire carrier. If you open up the budget a little you can make it really nice.
For the fuel mounts, I think you know a guy with plenty of scrap steel laying around and a welder.
I'm sure you kinda knew a lot of that. But just throwing it out there.
FWIW, when I have LP tanks or fuel cans, and my water jug in my bed, I put them in milk crates. A 5 gallon plastic can, a LP tank, and a plastic 5 gallon water jug all fit nicely in the standard milk crate. The crates stop them from tipping over and makes them easier to secure. You could get 2 or 3 milk crates and mount them somewhere. Not sure if you were planning on having those on the exterior or in the interior.
A buddy of mine through kayak fishing built this trailer that sounds similar to what you're building.
https://www.facebook.com/media/set/?set ... 239&type=1