Those who read what I did to my '86 Fifth Avenue may find this
somewhat interesting. I just took a 678 mile road trip to northern
Nevada via US 395 in the car, using cruise control, AC about 40% of
the time, speeds at posted limits not exceeding 65. Average max road
speed: around 62 MPH.

Vehicle/drive train: '86 M-body, LA roller cam 318, Holly "Carter
clone" 6280 feedback carb., A-904 trans with lockup converter. Basic
timing: 7° BTDC @ 630 RPM per California spec. Fuel: 87 average
octane "regular."

Trip plan: Lv Lancaster CA via north CA 14 to US 395 to the Nevada
state line at Topaz Lake and return. No local driving at destination
(just bicycling around the lake), no idling with AC on, no "warm up"
idling, etc.

Elevation at start and destination: 2580 and 5950 ft above sea level.
Maximum elevation: Conway Summit, 8130 ft. Route has three major
6-8% grades northbound.

Fuel economy going: 25.4 MPG
" " return: 28.9 MPG
Average: 27.2 MPG

All CA smog gear is working as per spec., as well, with no
disconnected EGR or other illegal mods.

The best mileage ever from this vehicle was a trip from Laughlin, NV
to Barstow, CA: 29.1 MPG, average max speed 55 MPH.

I think it's fixed. Why do newer, smaller models with V6s get worse?
One can only ponder, but the answer always comes back the same.