Yea that garage paint is high! Do what sound guy said.
Here in SC we dont have to get plates for trailers unless there a commercial one. Like if you have a lawn business and use a trailer or you pull your trailer for work filled with tools. I have 4 trailers (kindof, one is the inlaws thats residing at my house) two boat trailers, 4x8 utility, and my 16ft car hauler, none are tagged and i have pulled them by many law officers and logged many miles. You dont need a tag to be legal here. Thats not stopping you from getting one, but you dont need one!
As for the durability of treated wood. I have a say 18" hunk of treated 2x4. Its been a wheel chock for a boat i had untill i moved to the new house and it sat under a carport on concrete till i sold it. Anyway i used that same 2x4 (not a ground contact one just the normal deck 2x4) from the summer of 05ish to spring of 08 when i bought my new house. Now its still kicking around outside currently either in the wood shed on the ground or beside the woodshed. Its used to keep things like implements off the ground or keep a hay bail off the ground or a wheel chock or something. The center pithy part of the board has been eaten by termites adn a few other small galleries from the side that was always on the ground. But for being on the ground everyday for over 3 years half of that in a shady moist damp spot on the north side of a shaded gargage. It still, for its length probly as strong as an un chewed 2x4, it by no means will break if you hit something with it and is not crumbling if you squeeze it. This is just to show you how durable just a normal treated board can be, one on a trailer in the air where it can dry(not stay wet against the ground) will last 10x as long easy!