I am not sure what "having trouble filling" means in this case. If the vent were blocked, filling might be harder to do but the situation is not the same as a blocked vent on a fuel fill. The nozzle on a gas hose senses backflow pressure and automatically shuts off. The same is not true for a water hose filling the tank. The water pressure is a lot higher typically on a garden hose. A blocked vent on a water tank is more likely to cause problem when drawing water as the tank develops a negative pressure.
If the vent is blocked AND the filling hose or nozzle makes a very tight fit, then I might see how the flow could be reduced.
That is easy to check; just hold the water nozzle away from the fill an inch or so and squirt water directly into the fill. If most of the water backs up , it means one of two things; The fill hose has a blockage or maybe the tank is full ( if there is a combo fill, you won't see water overflowing from the vent, if no combo, water will vent out the external fitting which could be on the gunnel or on the hull side)