Short on power when towing

[BOYETTE]

02 f350 4x4. 410 gears 35 in tires runs great empty but u put even a bass boat behind it and it struggles to stay over 60-65 in overdrive. Just seems weak and egt,s will spike very fast I can hear air hissing when pulling in overdrive. Is this normal. It will build 18 -20 pounds boost when u floor it but while pulling it usually stays between 7-11. Is this normal ?? It has a banks six gun. U can tell the difference when u turn up the banks but still weak. Very disappointed in this truck Maybe there something wrong Hopefully something easy and cheap. What boost should this truck maintain while pulling ?? Any suggestions would be appreciated !