A little bit of water sitting in an open pan (the bottom of the heater) has plenty of expansion space upward and won't cause any damage. I do leave the plug out of mine after bypassing and winterizing the rest of the system, mostly just because the first thing I would do in the spring is to pull the plug for flushing the system.
I guess a little more water might slosh out on the final drive over to the storage lot, but I don't worry about the bit of water left in the tank, and doubt any of it will come back out the drainage hole due to freezing expansion.
2011 Jayco X19H (purchased 2015)
2008 Jayco 1007 PUP (purchased new, traded for the X19)
2018 Nissan Titan Midnight Ed.