Do you own an RV? If so, you'll need to make sure each tire is properly inflated before you hit the road. To determine the correct pressure, you'll need to know the load/weight of the vehicle. Here are some tips from Goodyear.com.

Weighing Your Recreational Vehicle
 
1) Your RV must be weighed fully loaded. That includes food, clothing, water, fuel, supplies and any towed vehicles.
2) Your RV must be level on the scale.
3) If possible, weigh the RV by individual wheel positions. If that's not possible, weigh it by individual axle positions.
4) A difference in weight from side to side may mean you need to redistribute the load.
5) The tire inflation pressure should be adjusted to handle the maximum load. All tires on the axle should carry the same inflation pressure.
 
Weighing Your Travel Trailer

1) Weigh the trailer fully loaded. That includes food, clothing, water, fuel, supplies and any towed vehicles.
2) First, weigh the trailer unhitched from the tow vehicle. The total weight must be less than or equal to the Gross Vehicle Weight Rating (GVWR) for safe operation. If it's not, remove contents until that weight is met.
3) Weigh the trailer by wheel position (preferred) while it's hitched to the tow vehicle.
4) While hitched, the tow vehicle should be weighed.
5) The proper hitch height is required for a level trailer.
6) Even distribution of the weight between the front and rear axles is required.
7) The tire inflation pressure should be adjusted to handle the maximum load.