@cmcnice, I hope you do not mind that I am answering your questions here where everyone can see the answers.
Eight summers later the fridge is working perfectly. Some winters I turn it off others I have left it on. It is nice having cold drinks in the garage.
1. I have a small ACR branded fuse panel after the battery switch, after the low voltage disconnect for house loads. The fridge is connected to that fuse panel. I likely added appropriately sized wire that I had on hand. This thread explains my dual battery setup. Not a typical starter and house battery but two AGM deep cycles using one, the other or both. On long trips I may use both for the fridge.
2. I screwed directly into the solid floor below the storage area. Worked great in my 230. Might not be a good idea in your 252. Removing carpet and glueing down boards should be fine.
3. I did not buy a trim kit. Guess I could 3D print the pieces now if I wanted to.
4. I used a pneumatic automotive body saw that I had bought to enlarge 6.5" speaker holes to 6x9. It worked great but just needing four straight cuts lots of tools would work. A grinder would be an excellent but messy option in skilled hands.
5. The manual has a wiring length to size chart. The rated current at 12 volts is 3 amps and I found the average to be closer to 1 amp. I do not remember what size wire I used but I would guess around 16 gauge pure copper which could handle a constant 3 amps up to 23 feet or the 15 amp fuse maximum up to 5 feet which is similar to the wire run. This wire size calculator could be helpful in determining what size wire to use
Wire Size Calculator
For home AC projects wire is sized based on standards that do not account for distance. I followed those standards wiring my home but for DC I prefer to properly size the wire based on length and current. I have started using BNTechGo silicone wire from Amazon for all my DC projects. It is really nice wire to work with and reasonably priced.
The fridge must be our most used boat mod. Always on, always packed with drinks and always being used. We keep drinks in the helm to restock the fridge.