To the OP, a couple of points.
Your rise in tire pressure cannot be attributed to change in altitude. Here is why. Normal atmospheric pressure at sea level is 14.7 psi. Most guages measure in psig, meaning the 14.7 has been zeroed out. If you were to fill your tire at sea level, and then magically transport your RV to deep space, the ultimate high altitude, where there is no pressure, then the maximum increase in air pressure in the tire would be 14.7. The 30 lb increase you originally mentioned is not due to changes in altitude.
You have mentioned you added some air, then you said you adjusted it again. Please take your rig to a truck stop with scales and weigh the axles. Use those weights and the inflation tables to know the proper inflation on the tire. It is entirely possible that I misinterpreted your post on this. It could be that 90 is a perfectly fine pressure for your front tires. However, the placard in the coach, and the max inflation pressure on the tire are not good substitutes for know the actual weight on the tire. Purists will say to weigh each tire, and that is absolutely the best. But knowing the axle weight is far better than assuming a weight for the axle.
