I've seen a couple of forums where folks have had apparently low gas pressure when using the trailer regulator and removing the grill regulator. I'm assuming that the grill works correctly when using the grill regulator and a 1 lb bottle?
It could be that the pressure drop in the line to the grill is too great to be able to run the gas over at low pressure. As a test you can try a shorter line from a regulated cylinder (say using one from a household gas grill) and see if that works.
You can also try installing a tee in the high pressure side of the rpod propane system and place the regulator at the grill, that's what most ppl do.