Fresh h2o backing up and overflowing fresh h2o holding tank

One other low-tech possibility -- if the flexible fill tubes run under the dinette seats, and if someone has put bulky cargo items into the storage hatch below the seat, and if that cargo bends the fill tube upward, then water may shoot out of that fill tube when you are trying to fill your tank, thus soaking your pants.

Remove the offending cargo and try it again after changing your pants.

Please don't ask how I know this.