I wonder if the instructions say that because the grill (under normal operating circumstances) is regulated as it comes out of the box. And the propane QC on your trailer is already regulated. The published grill instructions would predictably state NOT to use the grill on an RV because the RV QC is already regulated, and it wouldn't work (or at least not very well). But those instructions were written for a grill being used as it comes out of the box; you have modified the grill (by removing the regulator) and so the instructions (in this instance) may no longer be relevant.
Just a thought.
Many people have modified many grills to be used with the standard QC found on an RV by removing the factory installed regulators. I just haven't gotten to modding my Weber for this yet, but it will likely happen some day.
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!