Hi, Debbie, and welcome to CF!
We need more data to be of help:
Is the toilet above or below the waterline?
or fresh water
Is it "backing up" with clear water (from the feed) or with "used" water from the holding tank
If it is clear water and your toilet is below the water line, do you have a siphon valve in the feed water line? If so, the valve in the siphon break may be clogged or stuck. If you don't have a siphon break, you need one.
If it is "used" water, and you have sea water feed, then your sanitary hoses may be clogged/calcified -- this is quite likely to be true if they are 4 or more years old. You could also have a clogged/calcified Y valve.
Peggy's book, by the way, really is the "bible" of such things.