Our H26 drains faster than a toilet in express.
We also pull the vent plug, unscrew and push the valve plate down. Never had any problem.You can inspect the tank drain plate and gasket from the bottom of the trailer now that you have the boat out. Possibly something sticky got on the rubber gasket or something grew over that area. That's a big valve and it should drain real fast. There should be NO gaps or leaks at all. This must be a sealed, captured voluum of water and voids in this system can be a very dangerous condition.RayS/V Speedy