You may have a piece of tank blocking your dump exit. When the manufacturer cuts the hole for fittings sometimes the piece gets left in the tank. When this happens the result is what you describe. But since your rv is 8years old it seems that the problem should have been encountered before unless you just been lucky until now. Once you get the tank drained you can get an endoscope which attaches to your smart phone for around 40 to 50 bucks and look inside. If you find a piece of scrap inside you will have to remove the dump valve to try to pull it out.
Richard & Jill
2014 Flagstaff 832IKBS Classic Super Lite
2018 Silverado 1500 Crew Cab Z71 4WD All Star Edition
Camping since 1989, seasonal since 2000
Car Shredder Op/Tech, Scrap Metal Recycling - retired
I deserve hazard pay when camping with my relatives.