Welcome to the Escapee forums! There are blow out plugs available for RV water systems but they won't get all water out of the tank but do pretty good for the water lines. In all RVs that I am familiar wiith the water line leaves the tank such that about an inch of water remains in the tank after all that can be drained out has been.
We have experienced very few mouse problems over our years of RVing and when fulltime. I did use things to plug the space around the water, power, and sewer openiings, as discussed before, but just as important, I sealed all other openings in the RV as much as possible. Doing that not only helps to keep critters out, but it also makes the RV easier to keep warm in winter and cool in summer. You might be surprised by the number of penitrations through the floor and walls of the average RV that can pass air and many types of pest. For permanent sealing I used a combination of expanding foam and steel wool.
Have a VRV with a 26 gallon tank. I drain the tank by running the water pump, including the water heater. I also open the drain plug. In addition to making sure it is level or even tilted slightly toward the drain plug. I obviously am leaving some amount of water in the tank because it smells bad after a few months. I solve it easy enough with a bleach mixture. But I would like to figure out a way to completely remove the water. I've heard of pumping pressure into the tank. Any ideas?