That's correct, the 8 ga is to handle the amp draw of the fridge running on 12v.
I plan on doing the same, rewiring the camper to run on the 7-way trailer connector. Our fridge died and we replaced with a 2-way (got a deal on it) so will just run on propane while driving. I've got solar installed to keep the batteries charged so I think the 10-ga in the Ram factory harness will be plenty.
In this particular camper I have 2 20-lb LP tanks but only one battery, so it makes sense to run the fridge off of gas. I've been using gas for the fridge for many years in the TT so assuming the TC is the same.