Jim,
There is an inner and outer seal for the slide on your coach along the top and on both sides. The outer seal is not to hard to replace, but it will take several hours for the average do-it-yourselfer to accomplish, because it is so difficult to remove the old adhesive. The inner seal is much more difficult on most coaches because so much woodwork needs to be removed for access. So I recommend that you only change the outer seal, since this will repair virtually all leaks if done right.
The weather-strip material is available from Beaver Coach Sales parts department, Alliance Coach parts department, or even Monaco parts. but Monaco would be my last choice. You will need to measure the seal length because it is sold by the foot,and be sure to add a few inches to allow for the 45 degree cuts at the corners. You will have to cut the seal to fit.
Gerald