Just replaced all of the D seals around the side on my Denali 5th wheel. The OEM seals turned to mush, similar to window glazing. Was lucky, my local RV specialist had a coil of D seals that fit the original track. I have no idea why the OEM seals would turn to sticky mush. Only used 303 or silicone on the seals.
Trying to attach a picture of the D seals that I replaced all around my slide. It is a D seal and the OD is 1". It slides very nicely into the existing channel attached to the body of the slide.