Hello STJ.
I noticed you said for the first couple of years there was no smell
I also noticed you said always parked in a campground with full hookups.
AND that you did use the Black Tank flush regularly.
So, question, when hooked up to the to the septic system in the parks did you 1) have the black tank open all the time or 2)
did you just open it when the tank was full and then flush it and then shut it again?
Reason I am asking is it sounds like you might have the dreaded "poop pyramid"
from using the 1st method
IF you always opened the tank then even using the Black Tank Flush the trailer comes with will not wash away the solids and they will accumulate over time.
IF that is the case you will need to do some remedial work. You could try filling the black tank 3/4 full with water and some dish soap when you depart one campground for another, and letting the towing act as the agitator to help clean out the tank. Empty first thing at the new campground and then close it up and only empty it when it is full, even though you have full hookups. Also flush with ample amounts of water.
Some poop pyramids turn into a type of concrete the Romans would have been proud to build with and so that may not do it.
Hope for the best,
Good Luck