Some clarification from a tire engineer.
First off I am not going to address what the correct C
ressure is for your RV. We will assume you have read posts on how to learn that number. There are different guidelines for Trailers vs Motorhomes. I want to focus only on setting the CIP
Dr4Film is almost
correct except for is use and reference to a Temperature compensation chart.
was off a bit till they updated their tech page last year after my input. Also Wikipedia
definition was correct till I added a clarification aimed at RV owners. The 1Psi for 10 F is ok if your base inflation is near 40 psi but many RVs use 80 to 110 psi.
The correct "Rule of Thumb" to use is 2% for each 10F.
Tire pressure is not based on any laboratory standard temperature (some claim 70F) but is based on the tire not being warmed from either use i.e. being driven in previous two hours or from being in the Sun for previous two hours. Even partial sun can affect the reading.
Classical "Temperature in the Shade" is the "Ambient" tire engineers are talking about. Not Temperature in a theoretical laboratory.
So regarding your specific situation of setting the pressure when the tires and air is 50F. That would be fine and we would expect the pressure to increase by about 8% if the Ambient increased to 90F even without driving or Sun exposure. If you are seeing that much change in Ambient you may want to not consider the 50F time as normal set the pressure time. I would think checking more than an hour pre-trip is a bit early if you are going to be seeing such large temperature swings. I have a 10% margin built into my numbers fo am not concerned about a couple psi variation.
Dr4Film is correct when he says "The ONLY time to check CIP is FIRST thing in the morning BEFORE the days temp has had a chance to increase and BEFORE the sun has had a chance to shine on the tires and BEFORE you have used the vehicle.
However if you are driving from the campground on top of Pike's Peak and stop for lunch for two hours in the shade in Flagstaff where it is 90 and check your air you might find a change of a few psi. You could adjust your pressure before continuing to Phoenix where it is 120F but I don't bother to adjust inflation by the 1 to 3 psi variation I observe day to day. Too much work.
My personal CIP is 75/80 F/R on my Class-C MH. Both of these pressures are more than 10% above the minimum pressure needed to support the measured load on each tire so I have a "cushion". I usually wait till I am home and an getting ready for next trip before I adjust my inflation to my personal CIP
so watching my TPMS readings, I simply monitor the running inflation pressure which goes up and down as Ambient, driving and Sun exposure changes the inflation. My TPMS will warn me of air loss so all is good as I motor down the highway.