How to measure how much solar we need?

Sounds like the OP hasn't even taken their camper out camping yet, so it is impossible to know how many AH they'll use and need to replenish daily. Solar is a great DIY project and you'll get the most out of it by learning how every component contributes and can be maximized, including the batteries. So I suggest basically as pianotuna mentions as a rule of thumb => 100+ watts of solar per battery for a starting point, but build your system capable of expansion.

I installed solar before I ever took our TT out for the first time. I installed 280 watts of 12V panels and have only upgraded the 2 12V batteries to 2 6V GC batteries.