1996 Dakota 3/9 5 spd MT:
This is my daily driver, and tow vehicle. I have been driving this truck since January 2010 and been suffering the same problem since. I got this truck at 220k miles, and now has 360k miles.

It started as fuel starvation when towing, then have to replace the fuel pump soon after. It has evolved to towing anything sustained over 35 mph burns the pump. It doesn't matter if I'm towing a 23 ft 5th wheel camper, 18 ft fifth wheel flat bed trailer empty or with a car, or the 5x8 dinghy trailer empty. I can't tow and sustain speed without burning the pump.

I have replaced all the sensors. The exhaust is low restriction. The fuel rail has been blown with air, the injectors tested with momentary 9V and blowing through them. The engine has been replaced twice, and included fuel components. Timing chain and valve components are good, compression tests good.

Fuel lines: Upper half seemed a little restricted, so I swapped with one a little less restricted. Lower half line blows clear. The lower half is shorter and has less bends. The pressure regulator comes with the new module every time. There is no return line.

Electrical: Fuel line wires clipped short at plug. Ground wire connected to good solid body ground. On/Off/On switch on dash gives option for factory wire feed or alternate: Connect to Key On 10 gauge at column, to 20A auto reset breaker, to 30A On/Off/On switch, and 12 gauge wire back to the pump.

Last pump failed while running on alternate wire feed only for life of the pump. Previous pump lived on factory setting entirely.

The tank is clean. I have had the tank out 3 times, and cleaned thoroughly. I have taken the bed off once. Now i access the pump through a hatch in the bed. I shouldn't need a zipper for this problem.

From memory:
Specs on the pump: 192 liters per hour. 90 to 115 psi
Towing at 60 mph, 12 mph is less than 7 gal per hour.

Pumps are from Advance Auto, Global Automotive, AM Auto Parts ...

What am I missing?