class c house battery problems

Better find a mobile RV repair guy and have him look over your old rig. There should be a "house" battery that powers your interior lights, fresh water pump, furnace blower, appliance controls, and maybe a TV antenna signal amplifier. The house battery is usually charged by the truck "chassis" alternator while you are driving. If your rig is connected to 120vac camp power, or to a built in RV generator, there should be a thing called a converter/charger that powers you interior lights, etc., and keeps your house battery charged. Your "chassis" battery is the one that starts your engine and powers your headlights, radio, heater fan, signals, stop lights and running lights. It is charged by your chassis alternator (which also charges your house battery while driving). Try to learn about the 12 volt DC and 120 volt AC systems and how to maintain the batteries and check charging systems, very important. If you need to add a house battery, a new converter/charger and some related electrical/electronic parts, and pay for installation, it may cost you over $500. If you have an RV generator and it needs repair, that can cost you another $500 to $1000. Your 1985 RV is probably worth between $2000 and $4000. Adding/repairing the electrical systema will not increase the market value of your motorhome by much, depending on the overall mechanical and cosmetic condition of the rig. You may decide to try to sell your rig and save money to buy a newer rig that's ready to use. Don't ask!!

I have also seen both under the hood , 80’s class c.

On that vintage both batteries,house and chassis should be under the hood.

As you stand in front of the RV with the hood open looking to your left above headlight on a platform, that battery should be the house battery and the one on the right should be your chassis battery.

Did u look to see if your entrance steps can be lifted up via hinged lid ?

I've had a mh where the rv battery was behind an outside compartment door. I had another mh that when you opened the door to go inside there was a step that was removable, and under that step was where the rv battery was.

Engine area or outside somewhere.

"owner didnt remember how to work house battery.. cant see where to add them.."

Charging etc is automatic..Do you mean replace them?

Hope you paid very little because your post suggests a lack of maintenance probably on everything...