On a previous TT I replaced the mouse-hole setup with a 30 amp detachable shore power cord and here's what I can tell you. Photos are below. Some or all this may apply to 50 amps too. Cords can occasionally get wrapped up behind a mouse hole and only be partially pulled out - the manufacturers should be putting in a removable access to undo a trapped cord. On our first TT, we got the dealer to do this when we bought it.
The round mouse hole opening is a larger diameter than the detachable inlet housing. I found a mini cutting board at a local dollar store that was 1/8" thick. Even had a cross-hatch finish which was good for better adhesion of the caulk. Perfect! Used a jigsaw to cut a donut shaped adapter ring. I was able to access the interior side of the power inlet and screwed the adapter ring into some pieces of plywood behind the metal siding. Adapter ring and new inlet housing were sealed up with some Proflex caulk.
Note that the inlets are available in square or round and one may suit your application better. Hubbell also makes a 50 amp RV inlet which should be high quality but have no experience with one.
The black collar on a shore power cord (on Marinco at least) comes in a threaded version or 1/8 turn lock version. Have had both and the threaded one is a royal PITA. I found a new complete Marinco (Park Power) cord with the better locking collar for a really good price on ebay. Our current KZ TT came with a Conntek inlet and cord and has the threaded collar and the cord is now tucked away inside the TT somewhere for an extension cord (using a generator adapter) or for a spare if ever needed. After a couple of years the lid on the Conntek inlet would not stay shut and would flap in the wind while going down the road. Replaced it with a Marinco one also from ebay.
AFAIK, the Park Power, Conntek and Hubbell inlet housings all accept either the 1/8 turn locking or threaded collars.
The conversion kits that are available come with some parts that you probably don't need, like the 4x4" junction box. The power inlets are self-contained. You can use caulk instead of the gaskets. I found just the inlet on ebay, again, for a great price. If you can find just the inlet/housing, that's what I'd get. If you plan to buy a complete new cord, you obviously won't need the connector and boot that comes with the kits and could put the savings towards a new cord. If you buy a complete new cord, the cord's jacket will be factory sealed to the connector which is better than a retrofitted connector for keeping water out.
There are many offshore knock-offs on ebay and they should be avoided and make sure it's marked CSA or equivalent (not an issue with Park Power, Conntek or Hubbell of course).
After I did the conversion, there was a large cavity for the cord storage that was no longer needed. I made an accessible cover for it (accessible via an existing exterior hatch) and it became a great new storage spot for things that did't get used often. That was a great bonus that came out of the conversion.
When using a detachable power inlet, never connect it unless the power is turned off otherwise the contact surfaces will become pitted, leading to heat and a possible meltdown. Same thing applies for the plug at the shore power end. I saw a photo once of someone's power inlet that completely melted and nearly caused a fire. Not a pretty sight.
I recommend getting a 90 degree pigtail adapter to relieve the stress on the cord at the power inlet.
