To the OP ...
One thing that seems to be missing in this conversation is how much, how frequently, you rely on battery reserve alone. :h Unless I've missed it the only thing you've mentioned is being able to run your coffee maker ... if that's the case a single G27 can handle the task. The trick is to load support the trailer's battery with your truck idling, just as I do. Without it, the inverter under heavy load from a toaster or coffee maker is likely to alarm but with load support it won't come anywhere near close. Obviously this isn't a solution for those who regularly dry camp without shore power but for those of us who only intermittently do so and have limited requirements load support works just fine and certainly simplifies battery reserve requirements. Considering this, perhaps you can better outline just what you expect these batteries and inverter to actually power, and how often. :@