Tank sizes

[DonSue1982]

I have a 2019 Grand Design 351M toy hauler. It was supposed to have 1 black tank and 1 grey tank. After purchasing I learned they added a third tank for the rear 1/2 bath. Trying to find the capacity of the ranks

 

[wingnut60]

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.
Good luck.