Did you change the fill and vent hoses? The tank has to vent and it vents either to a through hull clam shell type of vent or it may use a combo fill/vent gas filler fitting. If there are two hoses attached to the filler assembly under the gunnel, you have a combo fill device.
That said, assuming you didn't have a filling issue before you replaced the hoses ( and assuming you replace the larger fill and vent hoses), you may have installed one or both with a downward loop that can allow liquid gas to pool. Gas can fill up the hoses but should 'self' drain. Often a person installs hoses that are a bit longer than the original and mistakenly pushes the extra under the deck. That may cause unwanted loops. So you need to take a look at the hoses and see if they need to be re-positioned. Note that for the clam shell, it is alos possible that splash from the sea gets into the vent fitting. If there is a downward loop, that water will also pool and block the vent. The goal is to place the vent hose so that if gas from the tank gets in it, the gas drains back into the tank and if water from outside gets in, it drains back out the clam shell vent.
