I used treated 6x6 lumber to make shims below the plain old lumber 4x6's on the houseboat trailer.
This was in '07, just got the boards from HD, they're just sitting in place...
This is 4 of the 12 shims I made, they progressively get taller with more angle to fit the deadrise-
This was a year later when I was assembling everything. The main boards on the frame
are already going silver...
This is a phone pic I took late last year while changing out bearings and races. The shims
held their color better, but notice the shim closest to the camera, there's a big chunk missing...
Almost all the shims are cracked, one not shown, half of it is gone.
--but the main boards are barely the worse for wear after 13 years. I never treated them with
anything either. This trailer's spent 99% of its time uncovered, out in the weather. The only time the boat
spent on top was to launch or retrieve. The boat spent the rest of its time in the water or on blocks.
Just based on my trailer, I would wholeheartedly recommend NOT using pressure treated lumber!