Poor Engine mileage

2003 Ford E350 Super duty stripped chassis van, 5.4L reg fuel, no burn or leaks.

Plugs have ~15,000 miles and new coil packs about 5,000.

Vehicle typically gets good mileage (15-20mpg) and pulls an F150 PU with no problems and still decent mileage.

Mileage fell off to 10.6, so replaced the original MAF sensor and it popped back up to 16.1, but after two tanks of fuel it has fallen back to around 12 on mostly open-road travels.

Ford diagnostics and "mechanics" found nothing out of spec, but when I tested using an Autel diag tool, I found two readings that are suspect, as follows:

O2B1S1 =.075 to.880 Volts (fluctuates at idle)
but
O2B1S2 = 0 volts and no change in reading with any throttle change or at idle.

As well, SHRTFTB1S1 = -1.6 to 4.7%
but
SHRTFTB1S2 = 99.2% solid reading (no change at all with throttle movement).

What sensor would be causing this, or what other sensor tests/readings could further isolate the faulty component?

There is no MIL indicated on the dash.