Advice needed on RV’s (old ones)

What ever you get, make sure you have a good supply of roof sealant. I swear those older models were designed to leak. We have an older one which was given to us and unless it's tarped while raining water comes in.
Also expect gas milage in the 10 to 15mpg range.

Personally, I'd suggest getting a camper van if all you're using it for is short trip tri stuff.

Repeat after me:  "There is no such thing as a cheap car." 

There is no such thing as a cheap car. 

If you plan to drive it, what would it take to make that $500 paperweight road-worthy?  New tires (probably $150 apiece), brakes (another $400+), transmission flush ($100), oil change, coolant change, battery, tune up, etc.  I would expect an additional $2000 (minimum) for all of the above, and more than this if you have to pay someone to do it.  Remember, the RV will have super-heavy-duty components, so you may not be able to find brake shoes (figure 4-wheel drum) at the local auto parts store.  Transmission overhaul would be another $1500. 

Mid-70's is probably a Dodge chassis with a 318 or 360 V-8 and 3-speed automatic transmission.  Expect 6-8 mpg and leisurely acceleration. 

If you already have a tow-vehicle, a tow-behind is probably the way to go.   A little more of a hassle to get hooked up, but then you don't have to haul it into town when you want to get groceries.  You also drive the tow vehicle regularly, so you're on top of any issues before you start the trip.  With a self-contained, you probably won't drive it enough to pick up the subtle changes in driving until something breaks on your trip (or you spend a couple hundred $$ a year to have it checked out regularly).  Check on insurance issues.  A tow-behind may be substantially cheaper to insure and/or license than a self-contained. 

If you can do all of the repairs yourself and are looking for another time-sink, then buy the beast.  If you're not inclined to wrench on it yourself, you probably won't save as much $$ as you think buying this over something newer. 

We looked at the total costs of taking a long RV trip compared to inexpensive hotels, and the cost was pretty close to the same EXCLUDING the capital cost of the RV.