To weigh your vehicle you will need to find a public weighbridge. Many transfer stations have a weighbridge and will weigh your vehicle for a small charge.
For the most accurate results the vehicle needs to loaded. For a motorhome I would recommend doing it while leaving for a trip away as this is likely when your motorhome will be the heaviest, ie water tank is full and fully stock with food and the like.
Often the suppliest way to weigh it is place all of the axles on the weighbridge for the first weight (total weight), then drive forward far enough to so the front axle is clear of the weighbridge only leaving the rear axle(s) for the second weight.
For the front axle weight, subtract the rear axle(s) weight from the the total weight.