Weigh stations are for commercial vehicles and as a private RV you do not have to stop. Ports of entry are different. You of course have to stop when crossing international borders. The customs officer can require you to pull over for a more complete inspection. There are some state agriculture inspection stations which require all to stop but these should have signage stating that.
The border patrol would not like it if you drive past their check points, even in a small car. State Inspection places, like California inspection stations that check for beetle bugs or worms being brought in from out of state, and for firewood that has to many foreign bugs in it, that is something that they normally stop all cars, trucks and RV's as well. California has a $13 Billion food crop to protect.
Weight stations, no those are only for commercial vehicles over a certain weight, unless the sign says that you must stop.