2004 f150 fx4.

Suspect a vacuum leak and I have a vacuum tester...but I can't for the life of me figure out which hose I should disconnect to actually use it?

 

Where ever you suspect the leak is.

Why do you suspect a leak? Perhaps we can narrow things down a bit and give you some locations to start at.

 

Well I changed out the cam phasers, timing chain, guides, tensioners, etc...
I had the cam phaser knock problem.

Since changing everything out my gas mileage has gone way down and it idles a little rough.

 

Use propane or some quick start? Spray in the area of potential leak and it will be drawn into the engine and RPM will rise slightly and momentarily.

If that was the case it is likely that you forgot to reconnect a line or knocked one off. Use of a vacuum line diagram might be more useful to just trace everything. This kind of issue is tough to chase with a vacuum gauge.