I am going to have to assume that you are talking about the house battery setup on your motorhome. If you are talking about the battery that is used to start the engine etc. you should go to a local Auto Parts Store and let them know what size engine you have, etc and they can get you the right battery for your motorhome.
The minimum house battery setup I would recommend for your motorhome would be two 6 volt deep cycle batteries hooked up in series. The ideal setup for your RV would be four 6 volt batteries hooked up in parallel. I would not recommend using only one 12 volt battery to power the house part of the RV. In all likelihood your motorhome battery compartment is setup to run two 6 volt deep cycle batteries to power the 12 volt system.
We have some great information on our website about RV batteries, so I am going to direct you to a couple of answers I gave to some previous questions that I have answered.
Hopefully you have gotten enough information to understand why it is important to have at least two 6 volt batteries to power the house portion of your motorhome. If any of our visitors have additional suggestions for you they can add them by clicking on the add a comment link located near the bottom of this page.
Everything-About-RVing.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com.