Increasing the domestic bank size can be a good idea as it can provide longer intervals between recharging.
New trays neatly welded in with batteries strapped down and terminals covered to comply with BSS standards.
Batteries can be balanced so charging currents and loads for each battery are even. It’s sometimes possible to increase the bank size using 6V Trojan, Crown or US Batteries. 4 6V batteries will generally fit in the same tray as 3 110AH.
Before even thinking about increasing a domestic battery bank you should make sure you have the means to recharge properly and be able to monitor properly with some sort of monitoring system. Also understanding how efficient various appliances and equipment is on board is a good idea so you can make the exisiting battery bank capacity as useful as possible.