Common issue--the lube with a few toilet fills of water will make a huge difference. Keep using per the bottle directions.
Assuming you have ruled out mechanical interference of the pull rod or cable.
After the first couple years mine got increasingly more difficult. I changed the valve and it only lasted a few months then began to get dufficult. I think my situation is that the valve is mounted on the tank horizontally.
Now I put the lube/water in after dumping then cycle the valve fast to spread the lube.
Camco 40246 TST Drain Valve Lube with Coco Oil - 16 oz
https://www.amazon.com/dp/B000EDSSG6..._XpA2ybB274C4F
Thetford RV Drain Valve Lubricant 15843, 24 oz. Bottle
https://www.amazon.com/dp/B000BGK2L0..._huA2ybD34A2PV