Vaugh,
I found a Solar Charge Controller that can manage two batteries:

http://www.ebay.com/itm/Dual-Duo-Bat...ht_4432wt_1162

Unfortunately, I have not yet found one that can charge two batteries AND manage a load. I have an old charge controller that has a Low Voltage Cuttoff (LVC) and was planning to reuse a portion of that circuit to manage the fan but it is not ideal. Is there such a thing as a Dual battery charger WITH lvc load system? Alternatively, could someone recommend a stand along LVC circuit for a small load?

Bumper - I like your idea of using a bigger battery in the trailer. The ability to recharge overnight would be very good if I were to accidently fly two days in a row. Heck, with a bigger battery, one can also run a small fridge to keep the post-flight beverages cool...