Welcome to the forum.
If that tank is meant to be curved, it would be the first I have ever seen. Beyond that, can't be of any help...have you tried finding the Fuzion Owners Forum?
EDIT: See that you found some owners discussion to help. Good luck, but basically, just cannot imagine any poly tank for RV water being shaped like that. Not enough bracing for the weight of the water it holds.
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