Look into the electric valves by DrainMaster. I put those on my Seville to get around a possible cable problem & to give me the ability to dump the grey tanks from inside.
Probably not the cheapest fix for sure & it will require you to have the tanks completely empty when you do it but it will give you the ability to dump without having to crawl under a slide.
Al & Linda
2013 GMC Denali 3500HD SRW, Crew Cab, Std bed, 6.6L Duramax, Allisons, PullRite Superglide, Triglide
2011 Crossroads Seville VF35CK11