It was stated in an article I read that the average RVer spends 19 days/year camping. Most of these days camping I would guess are hooked up to shore power. For this group, a single, 12V battery would probably be fine. As has been stated already, we all have different battery requirments based on our camping lifestyle.
We have been 98% dry camping since 2011-2016 when we had 4 6V Trojan AGM's installed in our TC , still lots of life when sold, and have just finished 1 year with 4 6V Interstate GC-2 (450AH),in the TT, no or little genny time averaging 90 nights/year(over 6 years). Based on research and our own real world experience, our present battery group, which cost $600 canadian, we hope to get 6 years( the average) before replacement which works out to 28 cents/day. We don't abuse our batteries but they are "rode hard and put away wet" every camping day. When you have this camping lifestyle and rely on your power storage, you come to appreciate that you have 4 6V batteries so there is little chance of being without battery power, never worrying about inverter low limit shut offs and the heavier construction and longer life of these deep cycle batteries.
Nice to hear all the honest, real life camping situations in these threads.
Dave