Toilet gasket not holding water. Need a temporary fix.

If you are talking about the flush blade seal, it is attached to the bottom of the bowl outlet. To replace it, you have to take the toilet out and disassemble the toilet, at least that is what I had to do for mine on a previous trailer. I did notice a mineral buildup on the seal, so maybe something that removes mineral deposits may help. On mine I did notice that the minerals were causing wear on the blade, so I replace it too.
If you want more information send me a PM