As I remember, the Terrys of that vintage were aluminum framed. The boxes are pretty simple. In general look for"
1. Soft spots in the floor.
2. Any sign of water damage in the ceiling.
3. Operation of the fridge. It should cool down in about 4 hours operating on gas.
4. Operation of all the other appliances including furnace, water heater, and water pump.
5. Check for any sign of leakage at the tanks.
6. If you can, get a manometer check on the propane system by a shop. If not sniff around for any leaks as best you can.
7. Check the age of the tires. If they are over 5 years old, replacement is coming due. If over 7 years, immediately replace.
8. Check the type and age of the batteries. They should be deep cycle not marine deep cycle/starting dual purpose batteries. In any case, after about 5 years, batteries get a bit iffy.