I tow with a genny, and and ATV in the box of my truck. To manage loading, I aim for 10%, because my trailer tows well at that number. Having the box loaded stabilizes the load slightly for sure. Tongue weight recommendations are what they are, partially due to the nature of vehicles to under steer when loaded correctly, and over steer, when not. A heavy load in the box changes that aspect. A balance of feel and proper numbers is the best way. IMO.
Note - This required me to recently add a WDH, (even after moving my 2 batteries to the back bumper) not because things were towing poorly, and not because the truck looked or felt overloaded. But simply because my axle weights were out of whack.
Or I could give you the standard RV.net advice. Go buy a 1 ton dually with snowplow package and diesel engine.
