Jim's right, but a plunger might work...
..IF the blockage is where you think it is. However, I doubt that's where it is...elbow fittings aren't any more prone to cloging than straight fittings...and rarely does anything that will clog an elbow OR a straight fitting coming into a tank ever make it that far before it clogs the hose. I strongly suspect your clog--if there is one--is MUCH closer to the toilet. But no matter where it is, a plunger is worth trying. However, do NOT use any chemical drain openers. If you have a manual Jabsco toilet, it may not be a clog at all. The "dry/flush" valves on those particular toilets are highly failure prone...and when they fail, the valve hangs up, creating back pressure in the "dry" mode that feels like a blockage in the head discharge line. Sometimes, it can manage to hang up and/or break off and prevent the toilet from flushing in either dry or flush mode. Find out if it's the toilet, and not a clog, disconnect the discharge hose from the toilet and use a hose (NOT the one you use to fill your fresh water tank!) with a garden hose nozzle on it to force water through it. If it goes through into the tank with no problem, the hose isn't clogged. And btw...I recommend doing this over a bucket or a big pan and stick the end of the hose in a trash bag first to keep water from going everywhere. Or, depending upon the age of the hoses, there may just be so much sea water calcium carbonate and urine crystal buildup in the hose that it's finally reduced the diamter to -0---or so close to it that you can't flush. Is water draining out of the bowl at all, even if it takes all night to drain a cupful? If it will, put about a cupful of C.P. Cleans Potties or K.O. holding tank treatment (both available from the online store here) in the bowl and wait a couple of days. Both products are bio-active, non-chemical--so they won't harm anything in the system--and "eat" clogs. But it takes time--at least overnight. Finally, check the entire discharge hose for a kink in it somewhere...I just mentioned in an earlier post that bending hose tighter than it wants to bend can result in a kink. The key solving this problem is in finding out for sure what's causing it...So let's exhaust all the easy possibilities before we do anything drastic that may turn out to be wasted effort.
