The Thetford Aqua Magic Style Plus model toilet is a porcelain model with a foot flush mechanism. If the toilet is no longer holding water in the bowl but the waste ball is closing fully, it is time for you to replace your 'waste ball seal'. You can locate this part below.
Thetford model numbers can be found on a small white decal at the rear base of your toilet. They are 5 digit numbers and pertain to the specifics of your model. If the number listed below matches that of your toilet, you have an Aqua Magic Style Plus and you can view/ order replacement parts based on that.
Thetford Aqua Magic Style Plus Model Numbers: