Thetford seal replacement

If you are talking about this one like mine after a good clean, then yes it will be very difficult to twist. You do have to create some leverage (I used a longer cut piece of wood between the cover slide) but it won't just break free, apply constant turning force and it will slowly turn all the way to the open arrow marking. Remember to lubricate before re-assembling, it's nearly as hard to do that.

Suggest you use the grey sliding cover to give you more leverage.  Just put in in the other way round and it should do the trick.  Did for me.

I have used an old number plate for some time to remove the main assembly in order to replace seal etc. As can be seen in photos, carefully cut/ file one side of number plate until it is a good sliding fit between grey plate sliding grooves. The number plate I have used is just the right thickness and gives good leverage, make sure the main assy. Is turned to line up the triangle symbol with the open round symbol on the cassette