Fully charge them and have them LOAD TESTED. Free at any place that sells batteries.
That will tell you if they are good/bad.
Much of "what batteries to use" depends on the configuration of your battery tray/area.
If it will accommodate them, 6 VDC golf cart batteries wires in SERIES to make a 12 VDC battery is usually the least expensive way to get quality deep cycle batteries. The come in different heights as well as other dimensions, so depends on what will fit.
As to how many/total amp-hrs, that really depends on how you use the coach. If mostly RV park to RV park VS lots of boondocking-- the answer will be different.