Be VERY careful if you do use bleach.
I had just cleaned only the 'bowl' with a spray bleach cleaner and didn't rinse it first and then put down my blank tank chemicals.....damned near died from the chemical reaction! :E
After I changed clothes I looked at the label of the black tank chems and sure enough it warns you not to get bleach anywhere near it! :B
If the previous owner did not use the blank tank properly by leaving the black tank hose open all the time while parked or not using enough water, yes the sides and/or bottom can be caked with poop which will make the smell come up into your RV when you flush.
Filling it up with just plain cold water to the top 'several' times and dumping will help clean it out. A bottle of lemon cleaner added will help also. Just make sure you rinse a couple of more times after.
I do this 'regularly' when I am at a CG which makes it real easy to do. I have never had any smells come from my black tank AND my sensors work all the time. :C
