you look to have at least 4 or more long square sections that go length ways to the camper
horizontally across the camper attach alloy (2.5mm plate) 100mm width (or at least 25mm wider each side then the fork plates) then 2 or 3 screws rivets to attach into each long section
this creates the structure to mount your plate section (front fork holders) some thing similar for the rear
try to mount the fork plates "over" one of the box sections 2 holes either side
for ease of fitment these can be mounted first off trailer
just my opinion