"(Shasta 2100 Revere - 1985)"
The Shasta is an old one, so what I am going to propose may not work.
You may not need a battery. My 15 year old Alpine Chalet had a converter/charger. It would supply 12 volt power when camper was plugged in. It did not require a battery at all. The Shasta may be the same.
Get make and model number of your built in charger. Do a Google search on those numbers. You may find an owners manual. If it is a converter/charger, the "converter" part will supply 12 volts for lights without a battery.
Otherwise, as posted above, any house battery will work for now.
I wish you good luck and happy trails ahead.