100 watts will work if there is no draw and could take a couple of days to fully charge depending on conditions. You will get approximately 5 amps output with a 100 watt panel Either get lithium with the panel or wait until your lead acids are dead then get the lithium batteries and the solar panel, if you just want to maintain your lead acids when the camper is not is use and not plugged in, the 100 watt solar panel would be just fine for that.
also if you buy a solar panel make sure is has the controller built into it then all you have to do is plug it and you will be good to go. Zamp panels is what i believe keystone uses.
https://www.zampsolar.com.
good luck let us know what you do and how it works