If you can get one side off the scale (Not likely at most scales) weigh one side marking where the outside of the tires are on the scale. Turn around and weigh the other side getting as close as possible to your marks. It does not take much leaning of the RV to one side to throw off your weights.
Moving companies, quarries, feed/grain operations, and land fills usually have scales. In OR they leave the scales on when not checking weights and I've found they are easy to get off to one side and usually have room to turn around.