I did this on a previous TT. One thing you may find is that the opening for the mouse hole is larger in diameter than the new power inlet housing. I found a 1/8" thick HDPE cutting board at a dollar store. Even had cross-hatching on it to help hold the caulking. Perfect! Used a jigsaw in a simple jig to cut the inner and outer circles. The inlet covers are available in round or square.
Def. go straight for the Marinco/Park Power kit. Beware of the knockoffs and cheapo equivalents out there. Hubbell has one that would be good too.
Instead of buying a 90 degree adapter, see if you can get a complete cord with the 90 degree connector. With some diligent shopping around, you may find one for around the same cost or not much more. Those adapters can be as much as a cord itself.
FWIW, there's two types of the locking collars - one is threaded and the other is 1/8 turn. I've found the 1/8 turn is much easier to use.
