This was a big benefit I found of adding solar - meaning I no longer had to worry about looking after the battery when the camper was unused - the solar would do it for me.
Consider whether you just want a small panel that you plug in while parked or something permanently fixed. A small panel may keep your battery trickle charged, but I suspect it won't do as good a job as a proper charge controller you get with a reasonable size panel (e.g. 80w). The latter charge controllers tend to work in multiple stages and will actively charge the battery when it is low, then maintain it carefully once charged.
If decide to permanently install solar on your camper roof then you might as well do it properly and get at least 80w - i.e. something that will do more than just trickle charge the battery, since whether you install 80w or 20w you will still need to fix it to the roof, run cables down through the camper and wire them up - i.e. the effort is pretty much the same for either, so makes sense to spend a bit more for the same installation effort and get a more useful solar set-up.
Check the truck camper university sticky thread - lots of info there from various people who installed solar.
Steve.
'07 Ford Ranger XLT Supercab diesel + '91 Shadow Cruiser - Sky Cruiser 1
'98 Jeep TJ 4.0
'15 Ford Fiesta ST
'09 Fiat Panda 1.2