Good advice above, especially for RVs that have not been used frequently. Another thing is to be sure you NEVER allow the black tank to remain empty (while in use or in storage). When you flush the tanks after emptying them and leave the tanks dry remaining solids (there are always some in there) dry up and become permanent fixtures that cause odors and sometimes even clogs. Replacing the seal is a good idea on an older unit but another practice should be to use LOTS of water when flushing the tank and NEVER empty it until it is nearly full. If you MUST empty it before then first fill it with water. It takes a LOT of liquid in the tank to flush away solids so a partly full tank just won't do the job.
Good luck / Skip