power falls off when towing trailer 1997 3/4 chevy 454 pickup 4 wheel drive

Hi Guys:

I have a problem on mt 1997 3/4 ton chevy dually automatic 4 wheel drive pick up

When I am towing my 27 foot inclosed trailer emry Istart ot & after a couple of miles the power falls off from 7o mph to 20 mph.

If I disconnect the trailer it runs fine all speeds.it is not spitting any black smoke.

I have the converters removed as they were plugged up & almost caught the truck on fire on my way to Mass last year.

It starts fine hot or cold & idles o.k.

Any suggestions would be greatly appreciated.

I have a new fuel filter to install.

I have tried nozzle cleaner & a little alcahol thinking there might be some water in the gas, Aquick response would be greatly appreciaterd
Thanks in advance