Something that could help... If possible, install a flush (or Tornado flush) such that it runs along that shelf blasting everything away. Or if that's not possible, a Tornado flush that is pointed right towards the shelf to hopefully do some blasting of that shelf area. This would be useful only when you dump, but at least it gets "the shelf" clean/empty (hopefully) each time you dump so that you are starting from scratch rather than having it build up.
Here is a link to an easy, and inexpensive, install:
https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/30315748.cfm
Note: This is different than simply "filling" your black waste tank (with perhaps your built in flush connection) and doing another flush. The Tornado flush actually sprays the water in the direction of your choosing (in this case, along the shelf or at the shelf). If nothing else, it also takes the place of having to stick "the wand" down the toilet (...which requires running a hose into your RV and playing around with the wand in the toilet inside the RV, and then having to carefully get it all back out of the RV which is all very painful).
Good luck!
Chris
