Adding second battery in new 5th wheel

I'm about to embark on a power-hog project (involving powering 12v electric motors) that will be used in non-hook-up places where I'll be limited in access to other power (eg, a generator). It could easily drain a single battery within a fairly short period, but two should leave me plenty enough afterward to get by for a while until I can recharge.

I know that regardless of this immediate need, I know I'll eventually be looking for longer battery use time anyway, so I'm ready to move forward on it now. I just need to convince myself that I won't have problems with the charging system or other side-effects that I need to know about.