How about a transfer pump sucking on the drain for the wet-dry vacuum cleaner? I have stuck a sump pump driving a hose into a wet-dry vac tub to empty the collected water to avoid carrying the tub of dirty water upstairs and out the door. The transfer pump driving a hose would be a lot more convenient. The control could be manual.Another option would be to have a transfer pump on the truck sucking the discharge into a drum or into an outside cleanout.
I was doing some more thinking... given a trap adapter at the wall, I could envision a line to snake through from a portable tub to the trap adapter. The snake would go thru the flex line. I was thinking maybe some 1-1/2 inch Spaflex clamped to a piece of brass 1.5 inch OD brass. Or even more robust and/or more flexible hose. The trap adapter contains the liquid, and the tub collects whatever comes back. This would be a tool for somebody who deals with this a fair amount, and where trap adapters at the wall are either the norm, or could be installed before the procedure.