At least one battery is OK, assuming you've insulated the connectors for the missing battery. I wouldn't want to rely on the tow vehicle's 12v
for running the refrigerator since some TVs shut off that power when you turn off the key, and of course you need a trailer battery to supply 12v
to the breakaway switch for that worst-case scenario.
I have a single Group24 battery in my '75 Argosy that has worked out fine. It's O'Reilly auto parts' house brand, from 2009, and we do a bit of boondocking (with a generator.) The 1975-vintage Dometic doesn't need any 12v
and I have quite a few LED replacement bulbs for lighting.