Just pulled the underbelly covering from my Eagle266RKS TT last week, to clean my black tank rinser. Based on what I found, there's plenty of room for a wider (front to back) black tank, but possibly not longer (side to side). Definitely NOT ROOM FOR A DEEPER TANK! The problem you're going to run into is mounting it. There is a hole for the BWT cut into the plywood "subfloor" (your heat ducts, elect. lines, and water lines run through the space between this subfloor and the top floor). There are aluminum mounting brackets screwed into the underside of that subfloor which hold the mounting flange on the black tank. You'd probably have to remove those mounting brackets, cut that hole in the subfloor to fit the new tank, insert the new tank (tilting it to fit it through the hole), and reinstall the mounting brackets. I have no idea if/how the tank attaches to the mounting brackets, since I couldn't see the top of the tank - where I suspect any attachments may be located.
It could
probably be done, but may not be a job for the average DIYer, like me. I'd suggest discussing it with your RV dealer's service tech before making any decisions.
In the end, purchasing a portable tote may be the best way to go.
Just my $.02