We have a 2016 Grand Design Momentum 350M so I can speak directly about a XLR. However, most of the toy haulers have similar functions.
We use the rear garage as a dinning room (we have the bottom bunk that turns into a dinette), game room, TV space, and bedroom for our son.
We don't have the 3rd A/C and it's been perfectly fine. We have the main A/C and the second A/C in the front master all ducted so they can support each other/the whole coach. There are 2 ducted A/C vents in the garage area and we have found it to be more than enough to use the space even in upper 90's in the Carolinas this July - no issues sleeping, eating, watching TV, or playing cards at the happy jack table/sofas under the upper bunk in the garage.
Our unit has a second restroom in the rear and the second sewer outlet is for it. We dump this tank after dumping the main tanks. Our rear tank catches the second toilet and rear sink. Yours will just depend on how it is configured.
Hope that helps. Happy hunting...