There's also no room up on the roof of the trailer for kayaks, maybe with no solar or A/C. They would also be mounted at an angle, the front of them catching the wind up at the trailers peak. Depending on the boats shape, it may cause an awful lot stress on the fiberglass.
Thule has brand specific truck cab rack feet, Yakima may well have the same. They work fine, you just need to pay attention to how close you let the kayaks/canoe get to the front of the camper for turns. A single boat can get very close, 2 not so much. I don't use a rear painter.