My first suggestion is to look for that label that is stuck somewhere on the wall of the interior of your RV.
The manufacture is and was required to place a label that listed the vehicles Empty and full weights and the tire pressures for the front and rear tires.
The trick is, everyone should have their RV weighed occasionally, so that they know how to adjust the tire pressures to compensate for the actual full vehicle weight distribution
Second, I would check my owners manual which should list the recommended tire pressures.
Third, if the manufacturer is still in business, contact their customer service department. Look it up on the web.
Finally, when all else fails, call the tire manufacturer and ask them what they suggest for the specific tires when used on a motorhome.