"2 many 2: Is it difficult to get to the back of the fridge? Sounds like I have such a Dometic, so I need to decide between your method, or adding a switch. I'm not very experienced with electrical stuff, but I did add a relay and switches to an old tube style Hammond organ. I didn't kill myself or break anything, so I figure I could tackle this"
I have a different model than you but it is a Dometic so it should be similar. Open the outside access door for the fridge, the black plastic junction box with all the wires is on the lower left. Pop off the cover and simply unplug the wire for the heating element.
On the lower right side, mine has the wiring diagram on the the burner cover. The circuit is indicated as: "T" "Heating Cable" and #10 light blue wire. Mine has a simple female spade connector that just pulls off of the connector terminal strip.
You could just leave it un-connected, I added an in-line fuse holder to that connector and attached the other end to the terminal strip. Anytime I want it connected I just put a 3amp fuse in the holder (I have never needed to)
X2 On the checklist
X2 Check your own batteries with a $5 hydrometer
PaulJ2: "It's possible they weren't completely charged. I did have it plugged in to shore power for a day prior to the trip. Maybe that, along with the 2.5 hour drive wasn't enough to fully charge them? Prior to that, they were at a storage area, but disconnected from the trailer"
If your converter charger is a "Smart" four stage charger one day plugged into shore power will almost get the batteries to full. If your converter is a a dumb charger, that only puts out 13.6 volts all the time. it could take a few days. Usually driving while hooked up to the vehicles charging system is VERY slow due to the wiring size in the cord. A 2.5 hour drive would hardly do anything.
Good luck