Yes, they need to de-winterize it to properly test the plumbing's ability to hold pressure.
When I PDI I use air pressure to bring the entire fresh water system up to around 70psi and leave it there for at least 15 minutes or so. IF I lose more than 5psi I have a leak somewhere that will drip water if is was under water pressure.
They also need to completely flood the black and gray tanks well up into the coach to check for leaks at the base of the shower, base of the toilet and every trap -so, yes, they have to de-winterize it in order to do a proper PDI.
The only added cost to de-winterize and re-winterize is 2.5 gals of antifreeze - the labor should be the same since they need to pressurize the system to do a PDI anyhow.
I would have them re-winterize it if you're in an area where it may drop below freezing anytime soon. If you're not familiar with how to de/re-winterize your new coach then ask them to show you how step-by-step during the walkthrough when take delivery. Enjoy!
94-Newmar Kountry Star 40-KSDP
Spartan/Cummins 8.3C-300HP/Allison 3060 WTEC-II/25yr RV Tech RVIA Certified/Onan-Cummins Certified