When the entire rig is on flat ground, how much vertical clearance do you have between the box and the 5er? Should be at least 6 inches all around. Some newer pickups - especially with the pimp-wheel options (e.g. 20-inchers) - have very tall bed rails and the only way to get the clearance right and still be towing flat is to install a lift kit on the trailer.
2015 Rockwood Signature UltraLite 8282WS Platinum, GY Marathon LRD, TST 507RV TPMS
2005 GMC 2500HD CCSB D/A, Curt E16, Prodigy P2, Garmin RV760LMT w/BC-20 b/u cam