Truth is that with dead battery still connected it is hard to tell without checking.
Most of dead batteries check as open circuit, so attaching truck parallel should not be a problem, but if the old battery makes short, it needs to be disconnected.
In my Fleetwood battery has disconnect relay with switch on the wall and relay itself on the wall behind cabinets.
A bit work to get to the relay, but if it has to be done, it has to be done.
If battery has no disconnect switch, you should be able to disconnect it at converter.
Same guess with battery compartment size. Too many variations. My Fleetwood fits 2 golf cart batteries with room to spare that I use for extension cords.
Lately I put long battery from sedan in it. The long battery will not fit into my Superduty diesel, but will fit into camper.
Bottom line, you never know for sure.