I've had it since 2008 but I've taken meticulous care of it and what goes down the drains. No food that doesn't go thru a screen, no toilet paper in the black tank. It's been stationary with the valve open on the gray tank(closed for draining black tank) for 3 yrs.
I am noticing that this a.m that it's drained down to 1/4 of the tank overnight.
I'll try the vent pipe clearing then bite the bullet and probably go for the "snake" up the gray outlet. It will be a little less messy at 1/4 of a tank(than 3/4) and I'll have a mobile dump tank ready to hook up asap when things start flowing again.