All coaches leak air. It's normal. I think if you can hold 90 psi for a week or two, that would be considered very good.
My coach loses all air pressure in about three days now (it used to hold 90 psi for a week). I've been looking for leaks - and need to do some more investigation, as so far I have not found anything. There's a lot of air line and fittings and valves to leak, and most are hard to get to to check. If you look at the manufacturer's specifications for a valve (any valve with an exhaust port) the specs will state that some leakage is acceptable (and they give a leakage amount over time). So add up all the acceptable leaks, and the small leaks from fittings and such, and you find your air tanks draining quickly.
You can spray all the hoses, fittings and valves with a mix of soapy water, look for the bubbles, and hopefully find and fix the biggest leaks, but there will always be leaks.