Slow draining black tank has been my concern with our 2014 HC. The only "cure" is to raise the nose. I've 'scoped' it, run a snake down the vent, filled it to almost over filled and to top it off, pulled the belly cover down and replaced the valve and while apart stuck my arm in the tank to check for debris. Yes, I did flush, high pressure clean and sanitize it well first). I just live with it, doing a good back flush with both the built in and a Valterra outlet end flusher about every other dump or so.
You can help that puny built in flusher by adding a Valterra outlet knife valve. This is how I ended doing a decent black tank flush:
-Dump the black tank first, then close that Valterra knife valve.
- Open the galley tank drain valve and allow that water to go into the black tank. Dump the black again by opening that knife valve
- Do the same with the bathroom tank
- If those gray tanks are only 1/3-1/2 full you can combine the water.
- Using the built in flusher will also help adding water, but that's IMO, only an aid
Then there is the possibility that the black tank valve isn't opening 100%. The cable end is only bent over at the valve termination instead of having a locking device (or that's how ours were made up until I changed the valve out
Traveling with my DW Susan, a couple poodlepups and our 2014 Montana High Country 343RL, being towed by a 6.7 2011 F250 CCLB using a B&W hitch and OEM puck system