The SeaLand seal is neoprene made up of two pieces. You can easily replace it with a new one or remove it and clean out between the two pieces. A stop gap fix is to hold the valve open then run a stick like a Popsicle or tongue depressor carefully between the two pieces. A very small brush works too. You will eventually have to pull the seal and do it out of the toilet or replace with new
There is the possibility that the ball itself is gouged, but that seldom happens but that too is pretty easy.
The toilet does come apart pretty easily. Turn off the water, pull the front removable Velcroed on cover off, loosen the gear drive clamp then remove the china top and you have full access to all of the 'innards'.
All of this is in the SeaLand owner's manual which should be still available on the 'net if you don't have a copy.
That toilet has its foibles but is a heluva lot better the the Dometic in our new 5er