Chevy & GMC Vans

HISTORY
1993 SAFARI AWD VAN 4.3L vortec, MFI , engine-W, 93,000mi
Since purchase (2015)
-complete tune up-wires, cap, rotor, all fluids incl all driveline/cases, all filters, coolant etc etc (5500-7000miles ago)
-got 21-23mpg highway, 19 otherwise (for about 5000miles)
-On a summer trip to sequoiaNP in desert, was getting ck eng light (O2), checked wires for O2 sensor. (no code below 55mph, light would come on over 55, then go out at 55)

this year... Since started driving in April, engine light is becoming more frequent ...43 ESC knock, 45 O2 sensor (rich)... Exhaust smell bad, mileage local driving is 13.5 mpg a 25% loss, tested knock sensor, O2, and wiring- voltages correct.
Observations:
-code45 rich (consistent), and code 43 esc knock (intermittent)- codes at approx 2mi down the road/3-7 minutes from cold start.
-voltage gauge seems low (under 50%)
-temp gauge is low 1/4 to 1/3
2017 may to present:
- NEW O2 sensor- no improvement
- Change OIL to 10-40, change filter...no improvement
- NEW Fuel Filter, fuel pressure test inconclusive - no improvement
- Clean throt body, remove and clean EGR, add screen...no improvement
- New EGR...no improvement
- New fuel pump-mileage drops to 12.5...no improvement
- New fuel regulator-mileage drops to 10mpg bad idle at low rpm, code 43 returns(intermittent)-no improvement
- no start, replaced starter- voltage improved slightly, all other problems unchanged
- New thermostat, checked ign module, engine grounds, coil, visable wiring- mpg back to 12.5, no other improvement
HELP/advise please ](*,)

Plugged converter? Temp control sensor? Distributor?

Hi, thanks for some ideas...have both temp sensor and sender (just purchased), and on my worklist. But why would distributor be the problem with fuel and mpg? and...How do I determine if the converter is plugged?
Thank you for the input!