You have probably heard about the advisability of learning the actual load on each tire position on your RV. This is many times called "4-Corner-Weight" in reference to the basic position of tires on vehicles, Right & Left, Front and Back. If you have a towable you may have one, two or even three axles so while the number 4 may not apply the concept is the same. You should learn the load on tires on each end of each axle.
If you get to a large RV convention such as FMCA events (check Conventions link) or Escapees SmartWeigh there are some companies that offer the special service and use special portable scales that can give the load on individual tire positions. Weigh To Go.
Also I understand that National Indoor RV Center (Dallas & Atlanta Locations) offers scale weighing.
You might check with your local Moving & Storage companies, Builder Supply and even Sand & Gravel pits.
Some of the above have extensive experience with RV and can offer suggested levels of inflation needed but others can simply tell you the scale reading. You do not need the accuracy of "Certified" scale as offered by CAT at truck stops as almost any heavy duty "truck scale is good enough for our needs.
But you ask how do i use the scale readings to learn the minimum inflation I need for my RV. That's where this RVWorkSheet comes in. Simply download the PDF file and print it off. (You should also save a copy for future use or to share with other RV friends.
The worksheet allows you to enter the various scale readings for your type of RV and with simple calculations you learn the heavy end of each axle.
Using the Load & Inflation tables for your tires as found HERE you check for the MINIMUM inflation required to support the heaviest loaded tire for each axle. If you can't find your exact brand or size tire in my list you can use the information provided by Goodyear, Bridgestone, Michelin as you will see that all the tables give very similar (+/- 5 psi) numbers for identical size, type and load Range tires.
You can read a number of my posts on how to do the final calculations in THIS group of posts. Some even show examples of how to do the calculations and adjustments to get the final PSI including a suggested "Reserve Load".