Here's another crazy idea...if the vent hose has a belly in it that could hold some fuel, the pressure of filling the tank could cause the simultaneous symptoms you are experiencing. But this would only be for a brief moment until the 'belly' flushes its contents. You still need to do something to address this if it's true, because you'll spew deisel every time you fill up until the belly clears itself out.
This would be a recurring problem if the dip in the vent hose was low enough to 'refill' itself when you tack or get in a seaway.
Hope this helps.