I wouldn't want to leave the caps off my drains and the valves open. Although it is probably unlikely, but I just don't want any mice crawling up into my tanks, or bug, or any other critter.
I suppose raising one side of the camper to help it drain better won't hurt anything, but you really do need to put some liquid back in when finished. As stated above, there is always some residue remaining, no matter how hard you try to clean it out. Let it dry out and, now you have concrete residue forming on the side of your tanks. That don't come off very easy.