Duncan,
I think we got confused when you stated the "RV" and trailer tongue weight in the same sentence. My mind immediately thought your RV was a trailer. Then I realized the Seneca is a motorhome and you're towing a trailer with the MH.
Since you're close to the South Bay Area, check out ML Solar in Campbell. They have great prices, and since you can drive there pretty easily, you can put the extra $$$ you save from shipping toward more or better equipment. I've bought from them before and they were good guys. They constantly have ads on Craig's list here in our area.
One rule of thumb for solar is to have at least 1-1.5 watts of solar for every amp-hour of battery capacity. I have 410 watts of solar for my 220 amp-hours of storage. Like you, I'm stuck with room for only 2 GC batteries in the current location, but want add 2 more.
If the refrigerator vent is "close" to the combiner box and the wiring run isn't too far to the charge controller and then on to batteries, then the vent is probably the easiest way to go. Of course the larger the wire used (though more expensive,) the less the wire run length a problem. But, use high voltage panels and it work fine.
Good luck,
Tom
