I looked at a Coachman with a rear toilet that was retrofitted with a pan in the floor with drain to where the spare would be. I layed down to look and found a 2 gln gas can strapped under the drain with no way to empty it without laying down and getting drenched. Had I bought the rig,I would have gotten a real grey water tank to put in it 's place. The problem with your rig is the drive shaft and differential are in that area below but I would think a hose could be rigged to run the water to a tank at the rear. Have you layed down to determine if that 's possible ? Since you boondock you could run a garden hose out into the desert from under the van or into a bucket.