I have the same problem with my 2008 Winnebago Adventurer 38J with the W24 chassis.
When I turn off the breaker to the converter, it's the same as shutting off the main breaker, or unplugging from shore power. Correct readings.
It's got something to do with the converter because if I just unplug it, the problem goes away also.
I just decided to live with it, now that I know what is causing it.
The 12 volts supplied by the converter must have a slight amount of voltage variation due to the AC to DC. The level sensors are very low voltage, and fluctuations are causing the error in readings.
Happy Glamping, Norman & Elna. 2008 Winnebago Adventurer 38J, W24, dozens of small thirsty ponies. Retired after 40 years wrenching on trucks! 2010 Ford Ranger toad with bicycles or KLR 650 in the back. Easy to spot an RVer, they always walk around with a screwdriver or wrench in one hand!