Trailer tires are supposed to be good for 6 years. Replacing at 3 is way overkill.
Go past 6 years and you might start to see radials pop and make bulges in your tread or might just have a complete blowout. As a youth I would run truck tires way way past the expiration date bc of money.
Tires will generate heat when driven from all the sidewall flexing. A short trip will not make the tire hot, but an all day drive will. Heat increases the psi and increases the likelyhood of a blowout if the tire has a weakness. On a hot day the psi could double. PSI is to be checked on the tire as cold / ambient temps(not right after a long drive), and the tire is meant to handle the extra psi that driving causes.
Flush out water and your should be good to go. Not a lot to go wrong with a trailer sitting if it does not have leaks or odors.
Hook up to city water for a few hours to make sure nothing will leak, or at least turn your water pump on for a few hours so the pipes are pressurized.
Run the absorption fridge to check for leaks. Check it for leaks, yellowish powder or stains will appear on the coils on the outside side if it has a leak, or it will just not cool properly, but if you suspect a leak then I would do a quick research on more details of how to check for leaks. The freezer should freeze.