we don't use pressure treated because it seems to split/crack more than just good old douglas fir. use stove headed galvanize bolts, and counter sink the board so the crowned head is a touch lower than the surface. trailer bunk carpet comes in a number of grades, spend a few more dollars on better carpet. i like to through bolt instead of lag screws from underneath. fit your uncarpeted bunks, install the bolts, then a first layer of carpet on top with just a few staples to hold it in place, then another layer that wraps the entire board (just like wrapping a christmas present) then attach to the trailer, with washers and double nuts (all galvanized). jam nuts prevent loosening when that trailer bounces down the road. buy an inexpensive electric stapler and you'll be set.