Meh, anything but massive rot and I'd probably consider it a good project (if I had the time).
Even if I had the time I wouldn't want to go thru rebuilding all the structure and a bunch of the interior.
Get it running, gut the drivetrain out and find an old pickup with a 6 banger and make a hot rod out of the mohos drivetrain!
That's what I'd do. Sounds like more fun.
Or get it running (assuming the drivetrain is solid, if not, tow the whole mess to the junkyard, based on how rotted you said), find a Moho with a blown engine/trans, that has a serviceable interior and structure. Prolly find that one for $300 too.
However, unless my finances dictated the only way I could travel is in an old cobbled together, might breakdown any minute, Moho, I wouldn't want something so suspect to travel the country in. I wouldn't be above fixing it up and selling it though.
03 Arctic Fox 860
07 Dodge 2500, Mega, 4wd, 5.9 G56, BD Triple Dog, SB ceramic clutch, Industrial Injection Dragonfly Injectors, GDP 2 micron filter, Firestone Bags, Big Wig swaybar, homemade stable loads, ATI viib damper and a BD Brake
If you have wanted to basically build your own RV from scratch, this would be something like a kit with many of the parts required. In terms of money spent vs. what the result would be worth at sale, it's almost certainly a losing proposition; but if it's a project you want to do for the sake of the project, or if you have some particular needs or wants that are not available in normal motorhomes, it might be a starting point (though there would probably be better ones).
In general, though, I recommend cutting your losses and moving on. Given what you paid, you stand a decently good chance of not doing to badly parting out the systems, assuming they're in working condition. You might also be able to sell it as is for about what you paid, at least in some parts of the country. Around here, even a pretty decrepit $300 RV may well get a taker for a deer hunting camp or similar.
I've fixed up terrible old coaches, too. As one of the above posters pointed out, you can easily spend more than you can buy a better used one for. If you enjoy doing that sort of stuff, it can be really satisfying bringing something like that back but it sure would make better sense to part it out. You should be able to get more than your $300 back BUT getting rid of the shell might be a problem. You might want to see if there is a scrapyard nearby that takes motorhomes and see if they will spring for the $300. RV's never do really make sense, they are just a lot of fun!