BBC

I have a 97 F150 4.6 towing a 4000 # travel trailer. Had been getting 10-12 mpg towing. Am now getting only 6-8 mpg. Ford dealers deny there is a problem because 'check engine light' is not on. Mechanic looked at it, checking fuel filter, vacuum leaks, etc. found nothing. All they did was reset computer. I also have a 6000# boat. Last time I towed it, last summer I got around 9 mpg. I heard something about a transmission sensor, could that be it? Help!