Batteries should always be of the same size/capacity/age. If not, the older/weaker/smaller one(s) will draw down the others.
I think most units with inverters have 2 or more batteries dedicated to the inverter only.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired