Norgen is the valve of choice used by Prevost. To answer your first questions, there is nothing stopping you from taking a new valve and replacing the O rings with the best O rings available for the application (assuming the ones supplied are not the best choise) and using Never Sieze or some other thread compound to ease disassembly. However, as a practical matter I changed mine at 10 years (an arbitrary number I chose) not because they all had failed, but because the leans were strarting to show up. I think ten years for less than $1000 worth of parts is a pretty cheap price. I like the Norgrens because if the systems are maintained they are extremely reliable and as simple as an anvil.

As to moisture, if our systems are well maintained the air will be dry. When I have seen moisture in the system the air dryer hasn't been serviced in forever and the auxiliary air compressor was left to maintain pressure but it did not have a dryer or moisture trap or if it did, nobody serviced it. I bleed tanks every 5000 miles and change the dryer element every 24 months (20,000 to 30,000 miles) and so far have never seen moisture in my tanks (except for the wet tank) and especially in my air system.