Starting problems - 92 Fleetwood (E-350) motorhome

I would use a multimeter and see what the state of charge is of the batteries you tried. I would also test the solenoid, either with an ohm meter or by bypassing it and see if the starter motor turns.

A 65 is bigger than a 34 but not a lot bigger. A group size 34 battery will start ANY engine if it's charged and not sulfated, but the bigger battery will last longer.