I'd put the money you'd spend on a battery monitor towards a marine battery isolator switch and a generator. You'll end up spending more money (on the generator), but you won't have to worry about how much charge is left in your batteries anymore. I think most of the battery monitors are only flashing lights to impress people touring your Airstream. When it comes down to measuring how much charge is left, the inverter alarm screaming because it is about to shut down is a pretty good indicator that there isn't much left.
I wasn't sure how much we'd use the generator (mainly got two Hondas to run the air conditioner, which is a must in the southwest, especially if you do any boondocking). However, I found even in cooler weather, we use one for about an hour in the morning and evening to run appliances at mealtime; and that keeps the batteries charged up.
By the way, thanks DEITZ645, for the FantasticFan contact info.