Nick,
What size is your single battery?
For a one-battery small motorhome, at the very least make that battery as large as possible that will fit into your battery space. Since your single battery is a 12V battery, put the largest "standard automotive size" battery that will fit into the spot where the battery goes ... that would be a Group 31 size battery.
Ideally the Group 31 size 12V battery you choose should be a "pure deep cycle" type of battery. This might be a bit difficult to find, as a standard pure deep cycle Group 31 battery can be rare ... but is easy to find if you hunt for an "AGM" Group 31 12V battery. An AGM battery does not have liquid acid in it. It is a "dry" battery, but is also quite expensive ... about twice as expensive as a standard liquid acid Group 31 12V battery.
At least use a Group 31 "marine/deep cycle" 12V liquid acid battery as can be found in most automotive stores. A single one of these should allow you to boondock camp for up to two days if you don't have a standard RV propane furnace turned ON during the daytime and if you have it turned ON only at a relatively low temperature nights. Also, to get 48 hours of RV use out of a
Group 31 12V battery you must start with it completely charged at the beginning.
The suggestion above from BFL13 is excellent, as then it makes it possible to carry several batteries along for longer boondocking sessions without having to increase the size of your RV's battery storage box.
Good luck ... boondocking is not too hard ... and it's fun!