My previous Open Range did this...lost nearly 20% of the water underway if tank was full. Lots of trailers do, as water tank is mounted lower under frame, but haven't heard of many MH's having same problem. May be because tanks are generally mounted higher and suspensions are generally smoother than the basic springs and no shocks of trailers, not sure. As stated, if you can find the overflow and cap it, or install a 90 degree valve, it may help. Just put a sticker next to your pump switch reminding you it's closed as it my also serve as the release vent when operating.
2018 THOR Chateau 35SF
Two Labs, Bugsie & Blondie