If the shower is not draining at all, and you have a place appropriate to dump it, open the gray water valve to make sure the tank is not full. If that is not the problem, I would add water to the point there is a few inches in the shower then use a toilet plunger vigorously until something gives.
If the shower is draining slowly, it could be that the vent is clogged or the drain pipe is partially clogged. I had a squirrel storing acorns, into the roof vent. Took a while to get those out.
Sanitizing the fresh water tank might help remove algae that could be interfering with the sensors (if that is the problem). Completely drain, sanitize the entire fresh water system with chlorine bleach, drain, refill, flush all lines, drain, refill.
2014 Silverado 1500 5.3L maximum trailering package (yes, I'm towing the 34')