I found most Ford dealers knew next to nothing about Max Payload or Max Tow packages. Doesn't sound like much has changed in 3 years. Keep in mind, most people buy trucks for the look or to use as a car with bigger trunk.
If you ever have the chance to compare a Heavy Payload and a Max Tow side by side, there's very little difference. With the Heavy Payload, you get 7-lug rims and one more leaf on rear springs that will take load at about 1.25" compression from zero-load level.
Heavy payload does add bigger trans cooler and the 9.75" rear end, but all EB's have the 9.75 already. Max tow get the trans cooler anyway.
I doubt seriously that with an 8,000-lb trailer and WD hitch you'll exceed the payload capacity of the truck. Unless of course you plan to do something like put a golf cart in the bed or something else that's 700+lbs.
More capable solution would be Max Tow with air bags if needed. We have a few of the Heavy Payload 150's at work. They ride rough enough when empty that you can't put a half-full drink in the cup holders.