[rickst29]

Power to run the fridge will be drawn from the "most attractive" of Voltage sources. The following background information is important, and deserves a post of it's own:

In normal TM operation on the road, the "12V" battery charge of the TV actually offers about 13.5V It's controlled by by the alternator (2 wire alternators) or an interconnected ECM (3 wire alternators, used in Subaru and etc.) The TM batteries offer only 12.8V (fully charged) while being discharged. But a problem happens immediately, as current begins to be drawn through the Bargman: The "power loss" increases with current, and much longer wire lengths (with more resistance) are involved in the TV "power source path".

Inside the TM, there is a #10 wire from the end of the Bargman (behind the fridge) to the load center. This competes with the wire from the TM battery string into the load center - whichever is longer loses the battle, with higher resistance.

But the next long segment is a problem only for the TV path: the Bargman cable itself, including length inside the TM and outside on the hitch, along with any "extra" length which is looped up in towing.

Finally, on the path to the TV Fuse box and 12V bus, we have the most important wire in the equation: The wires inside the TV itself, FROM the "Trailer Battery" fuse into the Trailer connector, and along the body from the under-dashboard trailer connector to the Bargman. Nearly every vehicle manufacturer makes this wire tiny, using "chassis wiring" rules instead of NEC "home wiring" rules.

Do not underestimate the damage of that last wire, within the TV! In my 2007 Toyota 4R, it was definitely #14 or smaller, even though the fuse allowed 30A. When you try and pull 10A through that wire, to run the Fridge, your 13.5V is dropped to 13.1V (or worse) at the TV connector. Lose another 0.3V in the Bargman cable, and cable-end to load-center wire segments, and the trailer battery becomes the more attractive power source.

In "typical" configurations, the TM house batteries will be drawn down to about 60% SOC before a majority of Fridge power requirements begin to be drawn from the TV.