Poor gas mileage

Stop on a slight incline, shift to "neutral", release the brakes, then see if the van creeps downhill on its own. If it does not we will need to discuss dragging brakes.

Actually, neither of those should cause poor fuel mileage. A leaking injector will bleed off too little volume to be noticed, but it can cause rough running at start-up until the gas clears out. The bigger concern is that fuel can wash down the cylinder past the piston rings and dilute the oil. You can find that by connecting your fuel pressure gauge and watching what happens when the van sits overnight. That pressure should hold for days, if not weeks.

The best approach is to use a scanner to view the fuel trim numbers. Those will show if the engine computer is requesting more fuel than normal, or if it is trying to reduce fuel, but without success. For those two conditions, the live data screen will show the readings for all the related sensors, then it is up to you to figure out which one does not look right. Normally too little or too much fuel will cause excessive emissions. Since yours passed the emissions test, a sticking brake was a better suspect than an engine problem.

Yep, if it passed smog it is doubtful it's internal, anything that ruins mileage would also cause it to fail a smog check.

What are you calling bad mileage? What is a "normal" day of driving for you? Did it just suddenly start getting worse mileage when the gas blends were changed for winter?