Check both fill and vent for obstructions, pinching, and correct routing. On my toy hauler the vent hose between tank and filler flange was pinched and routed with a low point that would fill with gas while filling. This caused the filler neck to burp and shut off nozzle.
Also, my fill hose was pinched and twisted as installed by factory. Once I fixed the problems my filling problems went away.
Something that can help filling issues is to pull trailer up onto a couple of leveling blocks/boards. This helped filling my trailer before I had a chance to investigate and fix the problems.