That's a very neat installation, Mike! I've been thinking about a way to install an easily accessed, waterproof outlet on my 1999 3124KS, one that also will eliminate having a dirty power cord flopping around in the interior's battery compartment when it's stored. Now if I can only figure out a good way to run the wiring from its present location to the rear of my TM.
A Google search enabled me to find a detailed description and photos of how the owners of "Natasha's Camping Site" installed their 30 amp. Marinco outlet in their popup trailer. This link is:
http://www.title-3.com/DetachPower.htm