You can use my MotorhomeRVtirepressurecalculator.
To be found in next map on my public map of skydrive.
Motorhome tire-pressure calculatorFirst download a spreadsheet before using it in Excell or OO Calc.
This is best done by RIGHTclicking and then choose Download from dropdownbox. The top 2 things dont use, cant handle some things or asks to log in with your hotmail acount.
For this you need weighed loads per axle or better 4 point weighing.
But in lack of that , you will have to do with the GAWR's
If you then also fill in the GVWR ( gross vehicle weight rating) , the spreadsheet also gives some extra info about load.
Also give the built of RV ( behind 1 or 2 axles and twinwheel axle)
If you cant work it out give it all here and I will procuce a picture in my answering post with the filled in spreadsheet.
Tires already give Loadindex 115/112 and you san E load wich stands for 80 psi reference pressure ( wich is not the maximum pressure).
But look this up at the sidewall for shure, LRE or "maximum load 2680 lbs AT 80 psi ( cold)", but somethimes described different.