Maybe a local RV repair shop has employees that are familiar with Class C's of the same era as yours. They could try all of the appliances and devices and show you how to operate them while making a list of what it needs and how much it will cost in parts and labor. Expect to pay them something for their time and effort. Learning to diagnose and fix problems with fridges, furnaces, water heaters, fresh water pumps, generators, converters, house batteries, 12 volt DC lights and other 12 volt devices such as appliance controls and TV antenna amplifiers will take time and much of it is beyond the average RV'ers skill set. You are ahead of the game IF you have correct type of new RV tires. Make sure that the engine, engine cooling system, transmission including entire brake system including flex hoses to front brakes, dash AC, and all lights are working correctly for the safety of you and yours and others on the highway.
Before you spend thousands on an old RV, be aware that it will only sell for $4000 to $5000.