100w will probably do it but you really haven't given us enough information.
What is the amp-hr rating of the current battery bank?
How long/How many/What wattage are the electrical devices you are running?
How many days in a row are you boondocking between time hooked up to shore power?
If the water pump runs 5 min/day and you have a couple 5w LED that run for an hour a day...100w solar is probably overkill.
If the place is lit up like Christmas with 30- 5w LED running from 5pm-Midnight...100w is going to be a bit light.
