I've used this and never had an issue with sticking valves.
TST RV Waste Holding Tank Drain Valve Lube - 16 Oz
I never felt like trying to crawl through or removing the underbelly. A whole lot easier.
2018 North Point 377RLBH, Roadmaster Comfort Ride, Dexter E-Z Flex Suspension, Flexair 21k Pinbox
2017 Ford F350 Lariat CC DRW LB 6.7, Air Lift 7500XL with Air Lift dual channel wireless