Welcome to the forum.
Since the specs on the website don't address the 2 tanks, best bet is to contact GD Customer Service? OR--get a water meter to attach to the hose and use it to fill the tank. This will give a good idea of each tank capacity.
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