Tinner12002,
Our tank valves are also "through the frame rail" and very difficult to get to without being "on your knees, upside down" which is not the posture any of us want to use at a dirty, wet dump station. So, like you're considering, I installed a door in the lower skirting on the trailer. It is easy to open, provides direct access to the valve handles and eliminates the "posturing at the dump station". There are some pictures of the door in a thread from back in 2014. You can find them in post #1 in this thread:
http://www.keystoneforums.com/forums...highlight=door