I think that depends on how much battery power you need and how much space you have to install them. If you are on the cord all the time, one battery is plenty. If you are boondocking, it just depends on how much you need for how long you need it.

 

I think boondocking will be the norm. I have 160 watts of solar panels.

 

We have 130. Going to go with four 27s. Also have generator and inverter/charger 100 amps charge rate so can recharge more quickly, if need be.