I carry one of those aluminum telescoping extension poles in my rig, and have one at home. I use the 303 on all the seals. For the upper, under the slide out toppers I have a bunch of old long knee high cotton socks. I'll take 3 or 4 and slide them over the end, one on top of the other, and duct tape the last one to the pole. Dip it into some soapy water to clean the seals, let em dry, and using some more dry socks, spray the socks with a good dose of 303 and wipe the socks/pole end along the seal. Acts just like a mini mop. Depending on how long your slide is you may get by with a long wooden broom handle.
Sounds like a pain in the arse, but the top seals don't need to be done as often as the sides, so it's no big deal.
I do my side and lower seals every month or so, and the top seals every 3 months or so.