Take a look at these gauges: http://www.rvgauge.com/support/rvtech.htm
Do they look like yours? Great West Vans is a customer of that company.
You should find instructions on how to calibrate the propane gauge if you have an owner manual for the gauge. The link above has some owner manuals if you can find a model number on yours or find one that looks the same on that site. It's possible that there has be a name change but the hardware is the same.