I Winterized ............. but then this happened.

Thetford makes a RV Toilet Seal and Lubricant for about $8. Not sure if it can revive an older seal or not. Not to be funny, but another option might be some high mileage oil. It has an additive that slightly swells rubber seals. I'd research that before trying it though.

My old camper had that problem, but it wasn't worth fixing for me. We camped year round and I would simply add a bunch of water (or antifreeze) to the bowl that it wouldn't complete leak out before the next trip.