I think this is an interesting approach, treating the tug as a "tender" (in boating terms) to provide any required support to the trailer. I agree that a pickup would fit this particular duty better (and could do black water as well as grey), but the SUV (or van) would work for greywater, and conversely for bringing fresh water (in a separate tank!) as a tanker to refill the trailer supply. I suppose one could support indefinite use of the trailer in an unserviced location (boondocking) without having to move it, using this approach.
I think the transfer pump is better located in the trailer (under the floor near the shower) so that it would not need to pull uphill on the suction side. Not having the pump in the tug means not having it available for emptying the transfer tank, but Suz was planning that as a gravity operation anyway.
To avoid excessive rear axle
load, I assume that anyone planning this on a large scale (large tanks) would always empty the tank(s) before hitching up. The importance of this would depend on the specific tug, trailer, and tank.
An alternative implementation could carry the tank on a hitch-mounted cargo platform, which would have to be removed (and carried on a roof rack, for instance) to hook up the trailer. This method could even handle black water (since it would never be in the interior), but would be limited in size by hitch load capacity.
My only significant reservation would be that if the tug-carried tank is the only tank reachable by the shower, not just for transfering from a trailer-mounted tank to the dump site, then the tug must be present and hookup up (by hose) whenever the shower is to be used. I assume that there would still be a small tank for the sink, for this reason.