Your typical RV battery shouldn't be charged must faster than about 20 amps. Two of them at 40 amps. (100 AH batteries / 5 = 20 using the most common rule of thumb). More than this and you should really have a charger with a battery temperature sensor.
Other factors to consider: are you going to be trying to get as quick a charge as possible as your routine operating mode? are you going to be using a genset to run the charger and, perhaps, the AC at the same time?
If you really don't need the fast charge and you don't run a lot of 12v
stuff in your trailer at the same time as you are trying to charge the batteries, a lower amp converter should work just fine. But if you have a habit of needing the fastest charge possible while running every light in the rig and watching as 12v
TV/VCR combo while the SO is running a computer on an inverter, then you probably need the bigger one.
But, I think, getting a size for the max recommended charge rate of your battery bank is probably the best way to go. Do make sure you get on that is 'intelligent' and can do two or three stage charging and battery maintenance. (intellipower with charge wizard or some of the better inverter/chargers).