First, determine if the current seals touch the sides and top of your slides. If one is a bedroom or closet slide, check the same seal on the bottom. The bottom seal on the kitchen slide is different. If the seal wiper does not touch the slide side, then you should consider ordering one of longer length. If it touches on one side but not the other, then the slide is not adjusted correctly. If it does not touch the top, then again the slide is not adjusted correctly and needs vertical adjustment.
There are two companies I know of that make the seal, Uni-Grip and Clear Seal. The one made by Uni-Grip will not last as the metal teeth are exposed and will rust out. I use this type the first time and only got a year out of the seal. The one made by Clear Seal has the teeth covered with rubber. Clear Seal has a rather large minimum order, but you can order small quantities of that seal from the following:
ParKin Accessories Inc.
2920 Gateway Dr
Elkhart, IN 46514
See the picture below for what the Clear Seal looks like. There is not much room to work installing the seal. Your fingers will be very sore. I used a wide flat pry bar to help press the seal onto the coach wall levering against the side (and bottom on the bedroom slide). The top is a different problem as the roof of the slides will not support any weight, and will flex easily. I placed a 1 x 4 on the roof to lever against but even that was difficult due to the flexing. Mostly for the top, I lay on the roof working the seal into place and squeezing with my fingers.
There is another seal on the slides. It is on the back of the slide outer wall and compresses to the side of the coach when the slide is closed. This seal is a peel and stick and much easier to install. GREAT CARE should be taken at the top corners of the slide with this seal. NOTE the hole in the coach wall the slide goes into has square corners. This outside seal should be installed so these top corners are completely covered when the slide is closed or the slide will leak (see picture below - this one leaked). Monaco installed these somewhat rounded rather than squaring them off. You can do the same but make sure the rounding is as close to the outside edge as possible so it will cover the wall corner.
Hope this is helpful to your project. If you have additional questions let me know.
2001 Monaco Diplomat 40' PDQ - 08 Honda CR-V