Blackwater Tank

Retired VSP wrote:
... My wife and I had even cleaned prior to the service and allowed both black and grey leavers to remain open letting the water drain as we used it.

Any thoughts on this...

Not sure how long you had the valves open but your black tank almost certainly was retaining water since there would be TP blocking the outlet pipe. Combine that with driving and sloshing things around and that could explain everything. I'll assume that you had all valves closed while you were driving since that's the law.

And never leave the black tank open unless you are dumping it or you'll end up with a pyramid of dried poop in your tank, which is what you indeed may already have.

I really don't know of an RV repair facility that wouldn't have it's own restrooms. Employees are assuredly told not to use a customer's restrooms. Even if an employee stepped out of line and used a toilet a single use wouldn't have put much in the tank. It is entirely possible that the shop was correct in that driving the rig after the tank had been drained would loosen some additional debris.
Would the service you had done require anyone to run any water? Things like de-winterizing, fixing any water lines, pumps, valves, tanks, water heaters, etc. would all require running all the water lines to remove any air in those lines after servicing. Even electrical repairs may have resulted in the techs checking that all systems were working including water pumps and electric valves.

The tank has baffles which may have held back sewage when static. When you moved the MH the sewage escaped the baffles in cornering and stops.

Or the service manager doesn't know his employees very well.

Ask if he has a security guard...it's happened before that the guards on RV Lots hangsout in the comfort of your RV...I'd say roll with it and use a good disinfectant on the potty!