Looking at the exploded diagram on this site suggests you'd have to get underneath the toilet assembly to release the bowl.
https://www.leisurespares.co.uk/brands/thetford/toilets-thetford/c250-toilet-range/c250s
So that's probably a non-starter.
Probably perseverance with a fresh and un-rounded screwdriver that's a perfect fit may help. Also consider a screwdriver but in a socket set with a long reach drive.
Once you DO get the thing out, replace it using fresh, stainless steel screws.
Frktfij
December 2024