The rubber seals all around the slide--and there are a lot of them--need to be frequently lubricated. I use Thetford Slid Out Rubber Seal Conditioner. I also use Boeshield T-9 Rust & Corrosion Protection on everything that slides like the screw drives under the slide and the large rod on the top left and right. An important part of the process is to remove anything (dirt, sticky stuff from previous sprayings, etc) off the sides of the slide. If you get rubber sealer on those big top rods, clean it off before treating with the lubricant. I live in very dry Colorado so it is part of my annual routine when putting the PC to bed for the winter.
The above mentioned rubber seal spray should be applied to ALL rubber seals--window/door frames, compartment seals, spare tire seal.
One time I took it to an RV service center with an excellent reputation (not CW) and paid good money for a spring start up that was to include the above described.....NOT!