I had a 100 gallon black tank and left it for months at a time! When I had to pull the tank to repair something else the walls of the tank were clean.
All the products for black tank are garbage, a waste of money or bad for the environment and septic systems. Some states are exploring banning it. A properly functioning system does not stink! Unless you go up on the roof and stick your nose in a vent.
http://www.trailerlife.com/uncategorized/new-law-would-ban-use-of-holding-tank-chemicals/
http://www.eplerwood.com/beta/images/Tomasso%20Grad%20Project_RV%20park%20sustainability_2010_XII.pdf
Bit of a disclaimer on this, If your product has dried or congealed, you will not be able to dump it right away, will need to get some water in there to rehydrate it. This can be done by simply using the tank and using extra water when flushing, so leaving more than 1/2 a tank might not be a good idea.
Don't take my word for this take some dried out doggy do and toss it in water to see what happens!
