Congratulations! - from another 1975 Dodge Sportsman class C owner.
First, Matt_Colie has given you some great advice. Especially the fuel lines - when I got my clipper, my mechanic did a smoke test on the fuel lines. He said they couldn't find a single leak... because smoke was pouring out of little pinholes from every inch of line. We replaced all the lines and were good to go.
Also, check your tire dates - RV tires will rot before they wear out. You don't want a tire blowing up your underneath.
As for the model, there are many many different types of class C motorhomes built on the Dodge Sportsman van chassis. Best bet is to post a photo of it. Mine is an American Clipper - it is all fiberglass, 21ft. Most others were standard aluminum siding. The "Sportsman" name is simply the van chassis - it should be a Sportsman B300 360 2bbl when you're looking for parts. The 2bbl is the carburetor - the 360 only came with a 2 barrel carburetor that year.
Check your VIN using
this vin decoder it will tell you what year your chassis is for sure (many RVs will be X model year, but the actual chassis and engine will be a year earlier - mine are both 1975).
The 360 is a great little engine. It's just enough to handle the RV, but not so much that it overheats.
As for cost of maintenance/repair/getting it back in shape, I heard a lot of the same arguments - it's not worth putting in more than you paid for it, you'll never get your money back out, you'll be lucky to sell it for what you paid for it, and so on. Here's my take:
Even if you paid $2,500 and spent an additional $20,000 fixing it up, you've only spent $22,500 on your class C. Any newer class C you buy for $22,500 will need work and repairs and maintenance, so you're still going to put out more money. So why not put the money in your retro RV and enjoy it to the fullest? Personally, I wouldn't trade my clipper in for any new RV out there today - it's a great little RV, well built, bright and light (they actually had windows back in those days - not these dark and dismal dungeons they make now), and it's so nostalgic and fun to be in.