First thing to do is drain the gas and put in some good old regular, methyl hydrate and ethanol may be OK for modern cars but it's not what your bus is designed to burn and premium of any kind is a waste of your money.
Then invest in a timing light,disconnect the hoses to the distributor and set your timing to 28 degrees at around 3000-3500 RPM (wherever the mark stops advancing as you rev the engine), put the hoses back on.
Now remove the vacuum advance line from the plenum and try sucking on it, it should hold a vacuum (a Mightyvac tool is best for this but your mouth will do for a test), if it doesn't hold a vacuum the EEC valve on the aircleaner or the advance pot on the distributor is NFG. Put a golf tee in the line to the EEC valve if it fails your test for now, if the distributor fails you need a new vacuum can.
Test your temp sensor 2 as outlined in your Bentley manual, or install your spare just for kicks (you do carry a spare one, right?).
A fuel pressure test wouldn't hurt either and confirm that you have no vacuum leaks or rotted hoses, L-jet can't stand leaks.
There is no magic trick or snakeoil additive for passing Aircare but it's easy if your bus is tuned properly, sounds like yours is struggling.
BTW disconnect and cap that EGR, as you describe it now you're sucking dirty unfiltered air from beneath the bus anytime you're close to full throttle, not good. It'll also cause a lean mixture and aid in the demise of your heads.
Good luck.