DJ -
A place called
VTE Warehouse sells power posts, terminals, terminal insulators, fuses and holders. I used them for parts when I re-wired the power lines inside the TC and wired power from the truck to the TC.
They have three versions of auxiliary power posts.
A
50A Auxiliary power connector that can be mounted through surfaces up to 3/8" (~$5 per post)

A
Brass Core Jumper Bushing (350A) (~$10 per bushing)

A
Through Panel Jumper Bushing (300A) where you use the plate it comes on as a drill guide (~$20).

An alternative to the metal liftgate dual contact plug and socket is the
Anderson PowerMod HP "A" series. Unlike the Anderson PowerPole connectors (great inline, in cabinet connectors), the HP series has a nice panel mount socket and plug. I've mounted this under the wing of my camper to provide alternator charging of the TC battery bank. Worked great. Can order online at
Mouser Electronics out of Mansfield, TX using the Anderson part number (~$25 ea).

You might consider adding a megafuse at the battery or a switch so the posts are not always powered, in case of a short. Can sometimes be found a local autoparts, marine supply or battery dealer.