I check Thule, and they seem to agree with Yakima (no solution). How stupid of GM to put such restrictions on their customers.
For a trailer this large, which certainly has enough strong frame in the rear, I would be tempted to get 4 of the bikes behind the trailer on a hitch mount carrier. Have a cable lock thru the bike frames, and the camper frame (or hitch), in case something were to come loose. I would then get a wireless rear camera system so you're not constantly wondering if the bikes are still with you. Rear camera would be nice to have anyway.
thebrakeman ('70), DW ('71), DD ('99), DD ('01), DD ('05)
2004 Surveyor SV261T (UltraLite Bunkhouse Hybrid)
2006 Mercury Mountaineer V8 AWD Premier
Equal-i-zer WDH (10k), Prodigy Brake Controller