Hi Duck and welcome to the RVUSA Forum. The easiest way would be to replace brand name for brand name. Doesn't necessarily have to be the same model, but the same manufacturer. That way you would be 99% sure of getting a match to the toilet tank seal. Or you can go with most any other RV toilet, as they are pretty much interchangeable.
There is no need to worry about changing toilets nor to stay with the same brand. The only thing that might be different would be the water line connection could need to be altered.