Thanks for reading Russ. This is a good question. I and others many times warn about not exceeding the max pressure of a tire or wheel but we are always talking about the pressure when the tire is "cold" and by that we don't mean you can cool your beer by setting it on the tire. What is meant by "cold pressure" is when a tire is at the same temperature as the surrounding air and not in direct sunlight. You know, the temperature in the shade.
Without going into the details of the "Gas Law" and looking at what is meaningful not just measurable, we find that using 2% pressure increase for each 10°F temperature increase works out pretty well for real life. This brings us to the obvious question of what is a "normal" temperature increase. Tire temperature will increase as you drive faster, or as you increase the sidewall flex by lowering the pressure or increasing the load. Being in the sun or near the exhaust pipe can also increase the temperature.
The normal temperature increase over ambient is in the 30°F to 60°F range so I would expect to see a 6psi to 12psi increase in a tire starting at 65 to 70 psi. Now if you checked in the early morning while the day was still cool and then were driving in the heat of the day when it was 20°F hotter then we might need to add another 4% so we might see a 16% increase.
If your normal cold inflation is 65 psi you could see a 10 to 12 psi increase on a hot day. In your question you were asking about a 40psi increase which is 150% of the cold inflation. If you saw that level of increase I would suspect there is a problem and that you have a bad gauge or the tire is very overloaded or you have excess moisture inside your tire because the shop did not drain and filter the air from their compressor.
Your increase from 65 to 85 sounds high. I only see about half that in my Class-C. I run about 10 psi over the inflation required for the actual load and I drive at 60 to 63 mph and I see about 8 psi increase.
Have you confirmed your gauge and TPMS readings are accurate?
Are you sure 65 is not leaving you overloaded? Remember you need to inflate to at least the minimum inflation shown for the heaviest tire on an axle and then I suggest at least a 10% increase for a reserve inflation factor.
Tire & wheel manufacturers account for a normal pressure increase as outlined above when they design tires & wheels, so you should not be concerned unless you are seeing a large increase in pressure. If you do see a pressure increase like 25% you need to find out why. Remember you should not be bleeding down your tires when they are hot.