This won't be much help, I suppose since I have a Scamp
with a Sealand Toilet but mine came off by removing the 4 bolts. On mine at least, there was gasket between the toilet and the holding tank top that was slightly stuck to both parts but it came right off with some judicious wiggling.
I suppose a previous owner might have put silicone sealant in there. In that case you may have to try to slip a putty knife in between the parts to cut at least some of the sealant.
I found a new gasket for it at Camper World so I suppose the gasket is a common item. Before I put mine back together, I installed some blind nuts on the inside of the holding tank just to make it easier to assemble/disassemble.
I'd try to find a manual for it and see how it's supposed to go together. I found a manual for my 1983 Scamp
Sealand toilet by Googling the net.
Good luck with it.