....our 2 front rubber spacers are spring-loaded (using 2 stainless-steel strips), so when the camper is lifted, and rubber is pressed against truck bed *bulkhead*, the rubber simply gives way to the direction of lift.
Things to consider using wood spacer:
-wood is unforgiving as a spacer, and in my opinion, more devastating to a truck's metal bulkhead IF camper tub slams into "the train of spacers"
-if the wood (or rubber) spacers are not the right dimension, will the rear portion of your camper tub (if you have a T-shaped camper tub) slam into the back of your truck bed wheel-wells ?????
Personally, I use a huge block of closed-cell compression foam (the genre of foam used to crate and package expensive and extremely heavy electronic equipment) that spans the entire space between our 2 front spring-loaded rubber bumpers. The wide foam block is appx 4.5 x 4.5 x 36 inches, and extends ~ 0.5 inches deeper than the rubber, as a "pre-shock" before the rubber is reached (if compression ever was to achieve this!). The closed-cell foam has a compression rate that will not damage the truck's metal bulkhead (so far).
I tested this on our driveway: 340 feet of ~34% slope will do it, and using an extendable inspector's mirror, could see the compression of about 0.25 inches (ie. the rubber spacers had not yet been contacted, but it was close).
Anyhow, this is my experience using experimentation with our particular camper/payload; your mileage may vary (YMMV).