When the weigh station is open, you're best to stay away. Neither the truckers nor the DOT employees would particularly appreciate you being there, generally speaking.
Some states/locations leave the scales running with the display visible on the outside when the weigh station is closed. If that's the case where you are, you should be able to drive up at that point and check the weight for free without causing any ire.
As was mentioned, CAT scales or the equivalent are also an option. While they're not too hard to find in most areas, there are some places (like where I am) where the nearest one is quite some distance away.
The door sticker is usually for the maximum axle loadings. Often on an E-450 the rears would be at 80 psi and the fronts a little lower, the exact amount varying somewhat from year to year as the chassis ratings have changed. It's often a good bet that a typical class C is close enough to the chassis load limit that these are not terrible numbers to go with, at least initially.