Ours has done that since we got the rig new. I have used plumber's grease to lubricate the seal by hand (use gloves). I have also used Thetford toilet seal and lubricant. I put it in the toilet at night and let it sit overnight. It worked well enough, but since we don't full time, the seal tends to dry out in storage. Here's what I used...
https://smile.amazon.com/Thetford-To...s%2C166&sr=8-3
I have heard of using olive oil and other things, but just don't use anything petroleum based. It will break down the seals even worse over time.