We had the On-board Onan in both our 2012 36RL and current 2016 39MB.
Running both AC Full Bore you can empty a 40# tank in approx 10 to 12 hours.
We have never ran ours for any extended period of time, normally just a few hours to get the inside temp down for a Hot Days travel.
The exhaust goes down and to the street side frame, then runs to the rear with a right angle exhausting out just past the Bed Room slide. That way you can put on a Genturi to exhaust above the roof.
Be advised the battery power you currently have won't run the ACs for very long, unless you have sufficient Solar feeding them.
If interested, I could post a couple of pics tomorrow when it's light outside to take a pic.
Michelle & Ann
2018 Chevy 3500HD Crew Cab High Country DRW, D/A, 2016 RW39MB, Dual ACs, Auto Level, Auto Sat Dish, Stack W/D, King Sleep #, 17.5" Sailuns w/Disc, MORryde IS & Pin, Comfort Ride Hitch, 5.5 Onan, Res Fridge & Induction Cook Top