The best method is to use a scale, try a CAT Scale. CAT scales are at truck stops, so easy on/off.
If you plan on traveling, load it like you would for a trip. Go to a scale and weigh the truck & trailer. Park the trailer and go back and weigh the truck. The difference is the weight of the trailer.
The weight would be:
axles weights
pin weight (pin weight is the difference of the rear axle weights of the truck)
add these 2 weights together and you have the total weight of the RV.
CAT scales cost $13.50 first weigh, 2nd weigh, tell them you want to reweigh, and it is $2.00.
Another way is to load the truck with everybody & everything in it, full fuel, etc... and go weigh the truck, then go back hook-up the RV and weigh both together.
If you just want the empty weight, on the entrance door jamb may be a sticker with the information. Also, on the left corner of the RV are stickers with some information.