Black Tank won't drain completely

[Umphl3tt]

We have 2013 3900FB Montana with 1 & 1/2 bath. The 1/2 bath toilet has a horrible odor. We have cleaned the tank multiple times, always use the drop in's, checked the vent with no relief. When cleaning the tank from inside with a flushing wand it appears the black tank is not draining completely. Any ideas?

[Javi]

Drain, pour 4-5 capfuls of Lemon scented Dawn into the tank the fill until you see bubbles rising in the drop tube of the toilet. Let sit for several days, then dump using a clear plastic coupling on the hose so you can see the water as it discharges. You'll be surprised at what comes out.

After you do this, fill the tank until you see water rising in the drop tube and drain, repeat until you see only clear water with no solids draining from the outlet.