The fill valve on our toilet had an intermittent leak. For the longest time we thought it was the shower door. I'm not sure why it wasn't leaking constantly, but I just replaced it anyway. As handn said, it is a good time to replace the gasket and maybe buy a spare. Unlike a house toilet the, gasket goes on the toilet, not the flange, before bolting it back down.
A large piece of doubled up aluminum foil can be used to seal off odors from the black tank while you are replacing the valve. Be careful not to over tighten the bolts.
2006 Safari SE FB
2000 F150 4.2L
2011 F250 6.2L