I've had the same problem with our 8280WS.
I removed some sections of the underbelly on ours and found that the vent line is routed under the slide room drive and below the level of the tank fill line. The vent line could not be re-routed to change this. The lowest section fills with water so the fill line becomes the vent...ends up that the tank is only about 3/4 full.
If you want to fill the water tank completely, I've used an air nozzle on my air compressor that I bring along and blow the vent line back into the tank.
Nights camped in 2013 - 55, 2014 - 105, 2015 - 63