For some years Coachmen rigged up a system that works well to keep the sewer hose attached--I have one of them. Whether it works for your unit depends on the exact layout etc.
The drain valve and connections are in an enclosed bay, and on the "floor" of this compartment directly underneath (well, directly underneath once an elbow is hooked on to the outlet) there's an assembly of standard sewer plumbing fittings: a rotating toilet flange, an elbow (and I guess a short length of pipe between the flange and the elbow to hook them up), a length of pipe sufficient for the hose length, and a screw-in cap. The cap is attached to the skirting of the motorhome via a string or cable so that, when screwed on, the whole works doesn't swing around too much when driving. Some bayonet fittings are too big to fit in the pipe if they have "ears" for finger grips that stick out much, but there are others...or one can cut off the projections.
I can see about taking a picture and posting it somewhat later today; I'm not sure the description is quite perfect. If you have the clearance underneath and a place to mount it, this is a handy and effective solution, and should be not too hard to put together as a DIY project.