Unless the parking area is marked for vehicles with trailers only, he has as much right to park there as anybody else. Yes, it frosts my nads as much as the next guy when I have to part 1/2 mile away because of such but that's the way it is.
BTW, I pay very well for the rights to use the ramp and fish cleaning station at a resort here. Best $$ I've spent on fishing. I still have to put up with idiots who have zero clue that there are others in the universe besides themselves, just not as many as at the public ramps.