Ford: 7.5 E350 1988 ford truck V8(Uhaul), neg terminal, test light

Question
I have email you before, but I may have lead you down the wrong path. My problem is. What will cause my truck to operate at low speed, say around
20mph. If I press on the the gas peddle all the way it will stall out. Also it will idle just fine, but as soon as I try to give it a lot of power while in the idle postion it will stall out.I also notice my battery will become weak. At first you indicated to remove the 02 sensor, which I did but that had no effect. This is what was replaced as indicated below

1. 02 sensor
2. distributor
3. fuel pump
4. fuel filter
5. alternator
6. starter
7. battery

What ever is limiting the power to my truck is also drianing power from my battery.

Some people are suggesting catalytic converter,but I think it must be some type of control module which affect both speed and drain on my battery.

Answer
lets first ck to see if you have a battery drain what i want you to do first is to remove the  neg battery cable .now with every thing off and all doors cloce  if you have a hood light discconect it . now take a test light and hook one end to the neg terminal on the battery and touch the test light end to the neg. cable end. see if you have a 12 volt draw.then touch the cable end  and the  thermal end  with the test light on it the light should go out after you touch it if not let me know.