The check valve you speak of is actually called a joker/choker valve. It is in the drain line, just after the flush bowl. It is a diaphram style rubber tube, cone'ish in shape for lack of better description, that will allow fluids etc to go one way, but is supposed to NOT let things back into the bowl. Assuming you have a better brand head, you should be able to replace this rubber cone, and not have any issues as you describe, until the rubber hardens up etc again, then you replace it again!
If you head is not able to have said valve replaced, then make sure you get a head that is and has parts readally availible, such as Raritans PHII.
Sounds pretty new. I would pump the holding tank and flush lots of water through the head as your pumping it out. Maybe it will clear what ever is stuck in the valve. Then if you have to take it apart you have cleared out most of the crud and you are just dealing with sea water. Then review the directions on what you can flush.