1) Definitely add a second battery if you have room
2) as for what kind, I'd recommend two 6 volt golf cart batteries wired in series for 12V. It is the lowest cost (initial cost) solution (~$80ea from Costco/Sams, + one extra 2/0 awg cable and a Flow-Rite fill kit). (I like this solution for the more usable capacity repeatedly)
3) or if you dont mind spending more and want maintenance free, then two 12V AGM batteries, wired in parallel (+ two 2/0 awg cables..)
There is tons of information out there about different batteries and it really depends how you want to use your RV (dry camp, minimize gen useage, add inverter to use 110V things, etc.)
There are also many more opinions than facts out there, which this is just one...
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