It's a shame for wasted space fore of the wheel wells when you don't have interior door access, but for those front sections that do have access doors I was thinking of a detachable box (plastic? one whole side open, with latches and weather seal) that goes into the truck first, then load the camper, open the access door, pull the box toward the camper knee wall compressing the weather seal and latch it (other half of the latch pre-installed on the camper exterior knee wall). That would keep the storage cleaner and dry. Don't forget to unlatch before camper removal LOL.
Maybe something similar behind wheel wells Mike? Depending on its configuration?