I also had a pressure guage, dash mounted and it ran in the 60-65 psi range also. Never in 15 years of ownership and 150,000 miles did I ever have to change a filter. The bus did sit for long periods of time without being run..
Putting a pressure gauge in after the filters will tell you how much pressure is going to the injectors. When the filters start to clog, the pressure drops. The gauge should be on the dash so you can check it while driving at highway speed. It drops to almost nothing at idle. If it starts to drop while at speed, you know you have a problem. Seeing it at normal pressure (about 65 psi on mine) is very reassuring. This is well worth having for anyone.