For the past year the water was retained in the bowl for only minutes. I was prepared to replace the seal when I came across a SeaLand toilet seal cleaner. It's about the size and shape of a tongue depressor, plastic and thin. On one side there's a small patch of what looks very similar to the 'hook' side of Velcro.
Apply the supplied, paste like material to the bristle side of the tool.
'Let the flush ball gently close, pinching the tool between the seal and the flush valve. Scrub aggressively around the entire seal 2 or 3 times. Open flush valve, remove the tool, invert and repeat with the bristle side opposite to first procedure.'
This cleaning resolved the seal leak problem. Simple, quick and effective.
According to a local RV supplier this tool is difficult to find as SeaLand may not be making it available.
Bob