Water backing up into the shower pan is gray water.
It is your "overflow" warning.
Gray water DOES smell and is often confused with black water (which smells worse BTW)
Your black tank most likely was not the problem.
Gray water almost always fills up before black.
There is a "technique" (technique only) that is way safer than dumping black or gray water on the ground. Big fine in most states if caught BTW.
If you buy an "end shutoff valve" like Amazon.com: Valterra T58 Twist-On Waste Valve: Automotive
, you can use the dump mast to transfer gray water into your black tank to extend your time before dumping. Obviously you must wait until the gray tank is nearly full when you do this to make sure the water flow is FROM the gray tank into the black tank.
The correct technique is:
1) Dump mast end valve CLOSED and capped
2) Open Gray water valve all the way
3) open black water valve slightly and allow the gray water to flow into the black tank
4) Close the black valve
5) close the gray valve
The MOST you will transfer using this "technique" is about 10-15 gallons; buying you a day or two on your spot (or Blue Boy trip). Remember the dump mast will be full of gray/black water when you do dump from the dump mast! Do not open the mast end valve until your hose is connected to the Blue Boy or dump station.