I think Larry has it right. Once you have replaced the tires you have an additional benefit. You know the exact history of every tire you are riding on.

All the tire manufacturers are consistent in their recommendations. They recommend at 6 years from installation to have a dealer do a detailed inspection frequently, and under no circumstances to run longer than 10 years from the date code.

What is invisible to you as someone who was not the original owner is any damage that might have occurred from running any or all of the tires at a low pressure. All tire companies also agree that if a tire has been run at 20% less than the specified pressure it is junk and should be replaced. Are you absolutely sure that a tire on your coach that should have been inflated to 100 PSI has not been run at 80 PSI? When you replace your tires you will know the answer to that. The correct pressures are found on the tire manufacturer's web sites and are based on the weight imposed on the tires. When you install the new tires, weigh the coach and determine the pressure you require at each position.