Welcome Lindewoman to irv2
Looks like someone put the wrong substance on your gaskets at one time. What you can do is get some telc power and pat it on as much of the gaskets as you can for over head you may need a long broom with towel. You should be able to stop the slide 1/2 way out so you can reach around the slide. I wouldn't spray slide spray on at this time just use telc for now to see if it helps. Also clean the inside face of metal molding around slide if its black from rubber.
on being a Newmar owner and come down to the Newmar forum and join in. Plenty to read up on and I can give you some KAMRA in place of your new member and you will be a Newmar forum member.
Enjoy the forums and post often.
I'am sending your question down to Newmar Forum also.