The base temperature is around 60 degrees Fahrenheit. The rule of thumb for automotive tires is 1 PSI change for every 10 degrees over or under that temperature. However, this is really based upon a percentage and an RV with 95 PSI tires isn't going to react the same way as a car with 32 PSI tires. So I'd think more along the lines of 3 PSI for every 10 degrees.
Either way, these are "cold" readings in that the vehicle should not be driven before testing. By monitoring my SmartTire system I've seen where tire temps can be as high as 140 degrees, which is 60 degrees higher than ambient temperature. In some cases the air in your tires can be as much as 25 PSI higher so don't check them after a hot run. The air inside the tires needs time to cool down before an accurate reading can be taken.
As mentioned earlier, weigh your coach in the fully loaded condition, check the manufacturer's tire pressure charts to see how much air you need, then fill them in the morning before you leave. Running a little extra doesn't hurt and gives you a safety cushion. Tires do leak down gradually over time and it's better to have them leak down "to" the required pressure than "from" it.