Black tank valve are notorious for have an occasional leak, stuff get caught up in the slide. If it is an always thing, could be a cable adjustment. I added a twist on gate valve so no surprises.
Garage heat, yep. 1 duct 35 ft away from the furnace. I carry a oil filled electric if I have people staying and I will be at powered sites.
Not sure if our compartents are the same, 2 -6 volts fit in the upper tray. I added an invertor in the pass thru near the batteries and added a receptacle behind the TV from the invertor. Your floor plan is differnt but a bit of ingenuity will get you there.
Nice looking rig.
2019 Duramax dually
2015 Fuzion 325