I had that problem a few years ago where the pump wouldn't turn off when de-winterizing but it turned out a bit different than yours. While the water pump was running, I walked around the FW and found water was "running" out the driver side front corner near the jack. Turns out, I messed up the prior fall and must have left some water in the line running under the shower. I pulled up the pan and, lo and behold, there was a cracked pipe. Was lucky enough to get it fixed the same day since it was early in the season and the dealer who did the repair was not all that busy. Good thing yours started working properly without any problems.
2012 GMC 2500HD D/A Crewcab
2012 Jayco Eagle SuperLite HT 26.5 RLS
2009 Crestliner Superhawk 1900
2013 Polaris Sportsman 550 EXP
2008 Polaris Sportsman 700 X2