Found a place that sells a "kit" for building your own floor scale. My kit can handle 20K. The kit has all the guts you need, I need to provide my own platform. They recommend 3/4 or 1" aluminum. ...
I would suggest building three dummy platforms so that you can keep the coach level while weighing each corner. Having it tilted an inch or two in varying directions will lead to inaccuracy. With a little care in planning, the dummy platforms could just be a couple 2x10's stacked or something equally straightforward.