Agreed. Put another way, The WDH is only used to distribute the trailer's tongue weight to the tow vehicle's front axle and also the trailer's axles. Anything in the truck should not be taken into consideration.
If the payload in your truck is enough to cause you some heartache because the truck is sagging in the rear, you could consider air bags on the truck to "help level" the truck. Just take care not to use the WDH to "level" the truck.
'04 Dodge 3500, 5.9, 4x4, Quad cab
'74 Ford Bronco - currently frame off
'03 Outback 25RSS, SOLD, flipped axles (great dry camping trailer!)
'11 Lacrosse 296 BHS, 2- 6V batteries