Be interesting to see what you do.
A. In Theory
As long as additional batteries are hooked up in parallel to coach batteries, should not have to do anything else.
But,
1. Location of additional batteries may create issues (voltage drop, uneven charging?) as different length battery cables.
2. Coach battery charger is dinky 10 amp charger so would take even longer to recharge bigger battery bank (assuming greater discharge).
3. Central computer console should not be affected as probably reading voltage off battery charger (not sure).
B. Locations for Addtl Battery(ies)
Lots of different locations but will still have issue of 2 batteries side by side in water tank and one or more batteries set apart.
There is a hole through water tank between the two existing batteries--one could run cables to an under-chassis mounted battery.
Or if added batteries on top of battery compartment lid, would be close by but occupy space and raise center of gravity.
Ideal to have same size batteries, matched for age as well.
That may limit Group 31 locations (under kitchen sink cabinet area).
C. Relocation of Existing Batteries and Addition of New Batteries
Could put all batteries in new location under the body. Lots of discussion over on other subforms.
Locations include where our generator is (where spare tire originally was--between bunper and axle), alongside driveshaft (may need to be careful with Espar heater location), etc.
Why??
a. Lower center of gravity (use original battery compartment for lightweight storage)
b. Keep all batteries together if more than 2 batteries.
c. German website folks have had coach batteries weight cause splits in unreplaceable and difficult-to-repair fresh water tank.
D. Replacement with Lithium Batteries
Compare cost of additional battery and installation framework with using lithium batteries.
Lithium batteries allow greater discharge, more cycles, and more amp discharge for heavy loads
Lighter weight too.
Those advantages may address reason for more battery capacity.
Disads include much higher cost, different battery charger, no cold temps (but water tank location moderates temps).
E. Solar/Other Recharging
If need for addtl battery is not bec. Of heavy load amp discharge (e.g., microwave, etc.) but primarily lasting longer in one spot without hookups, then maybe look into solar.
Or if willing to use generator, a bigger battery charger with type of batteries which can accept higher charging rate for shorter period of time (not gels).