Look for back posts on this repair. Some have come up with alternate less expensive seal material. This fifteen page document explains how to break the Halves of the HiLo apart using a floor jack and 2x4s. I got my seal from J&R in Ohio. For my 25ft Classic it was $400.00 plus shipping. JR is a repair place in Ohio. These folks worked at the original HILo manufacturing facility.
www.cable replacement in my 1998 HiLo 24TD 3-3-2010. The material you are looking for is 3/4 hollow p bulb seal with 1" stem. Hopefully some one with more computer expertise can send you some links.FYI I used pipe insulation when I was set up in the campground to fill in the gaps on the outside. We have since replaced our seal and it 94% keeps out the dirt. Then I cut up a towel and made home made draft stopers on the inside. I filled them with beans. One I made is so heavy i can hardly pick it up. DH does this for me. Replacing the seals can be a do it yourself job if you are handy.