Since it is both tanks then it must be something they have in common. One thing you might try is to have someone inside the coach flush the toilet and hold it open while you drain one of the tanks. If there is a big difference in flow then it's likely you have a plugged vent line. If there is no difference then check the outlet where the two tanks connect for blockage. If you find nothing there then the only thing left is the hose and fittings themselves. If everything is clear then I guess it could be the design of your system but I think that's highly unlikely. Like Ramblin said, for most of us, before you open either valve you better be sure everything is hooked up nice and secure.
2013 Winnebago Aspect 27K
E450, V10, CRV, Blue Ox