Batteries in Parallel

Hi all, I have a Diamond in the Rough that has two 12 v batteries running in paralllel and a 100w solar panel for charging. The batteries were new last summer and they were trickle-charged in our garage over the winter. We took it out overnight and I found that the voltage meter was reading under 12 volts and our Norcold wasn't keeping cold. It hadn't occurred to me to connect them in series Instead of in parallel. Would that have done the trick? Thanks in advance.