Being the cheap (read that as fiscally conservative) guy on the block I wonder. . . .
Has anyone tried taking the 12 volt battery
out of an old computer UPS and connecting it to the trailer battery
via an isolation switch? I have seen several used UPS systems with a bad battery
for less than $20.00 that will provide 1000 Watts of computer grade 120 volt power. The system is all set to recharge the battery when on shore power but would automatically be on when not connected to the AC power grid - thus the isolation switch.