You are "on the road" as you said, so that is where you will weigh it at a Cat Scale.
Its easy cheap and nothing to be afraid or intimidated by.
First make sure you have a full tank of gas and that your water and grey and black tanks are at the levels you want them weighed at. Normally I would say fresh water at half or 3/4... Whatever you normally drive down the road with.
When you weigh your coach, you want it at your heaviest, normal situation driving down the road.
Step one - find a truck stop with a Cat Scale and if its empty just drive on it.
Step two - IMPORTANT... make sure each of your three axels are on a separate scale (square) on the platform.
Step three - walk into the entrance by the semi-trucks fueling area and tell the attendant at the register you "want to weigh your RV on the Cat Scale"
they will say "drive on"
You say.... "I did, and I'm on the scale right now"
At this point you can either pay before you drive off the scale OR drive off and park then come back in pay and get your print out. (mine was $13 just weeks ago)
Either way you do it, you can't get your weight printout till your coach is off the scales.
Bingo!!! less than 10 min and you have all the info you need!
Really its that easy and then follow the steps for your tire size and brand on their website. Based on your weight always go one tier above your listed weight so if your weight on your front axle is 13000 lbs, divide it in half ... so 6500lbs then if tire pressure by the manufacturer is listed like this....
6300 lbs at 100 PSI
6450 lbs at 105 PSI
6600 lbs at 110 PSI
6750 lbs at 115 PSI
You should choose the 6600 at 110 because it is the one over your actual weight.
Hope this helps!!!!