Charlie,
If you elect to go the cam/intake/header route suggested by falcon, selecting a good mid-range rpm cam for towing then replace your rear gears to put you in the sweet spot on rpm's at your cruising speed while towing. Once you've done that, a Gear Vendors unit will double your gear selection options, splitting each gear to let you pick & choose to stay in your power band easier. I know that John Norton used one on his bus, and I believe he'll say it's a good option for maximizing your power band with heavier loads.