Jim,
You're running into the same problem I have. The cabinet sides from the old Dometic 2 way installation don't leave enough space for heat to dissipate. I've used fans but it's still an energy hog. The sides stay very warm. I guess the heat "soaks" back through and starts another cooling cycle, which produces more heat and....
I'm still thinking and looking. By the way, one 120 watt mono panel and two batteries won't keep up- even with a fair amount of driving. With the same electrical set up, I used to run a small generator about once a week. Now it's no less than every other day for several hours.
I use a Harbor Freight 1000 watt continuous inverter. The first one died in a week but the second works fine. It seems to run about the same on high line power as the inverter. Often!
fleamarketvendor