I would be working out exactly what I want to run off the batteries/ battery.. The average amp draw, then what form of charging system you want. Solar.., generator,. Etc... How much power you can put back into the system while away.
Then decide will that cover the average length of a trip away for you...
That should tell you how many amps you need to "carry"
Once that's been assessed, then I normally just explain ... It's a safety issue... Gotta have a second battery on the LHS of the camper to balance the weight for the cool gismo thingymajig I got last week and fitted to the right hand side...
It's all about safety... Honest.