I should have noticed when you said two toilets on the same black tank. I can't say that I've ever heard of that. Usually when you have a bath + a half bath, they will have separate black tanks. Looking at the spec for your 415 AMP, it says 108 gal tank capacity and only 45 gal gray capacity. If that's true, they are probably putting gray water from the vanity in the half bath also into that black tank. I'm surprised that there isn't 2 separate gray tanks...one for the galley and the other for the shower/vanity in the full bath. As you're having problems with the full bath black tank, it could be venting on it or some blockage on the toilet drop. The toilet will most likely have a couple of 45 elbows in the drop to line up with the tank and potentially more problems than a straight drop.
Nights camped in 2013 - 55, 2014 - 105, 2015 - 63