I'm not sure what the relief valve is that you're talking about is....and the return line? is that the return fuel line you're speaking of?
Have you located the vent line?
What size is your fill line....it would have to be very small to have a restricted vent effect the filling as the tank will vent through the fill pipe when filling.
My
money is on a restriction in the fill line....I have never done this but I would think a
plumbing snake could be used to probe the fill line.