As you say you will be close, so if you are dead set on not being one pound over, than you need to go with a gasser to get the extra payload.
One thing you haven't figured is the 475 pounds of tongue load on a long hitch extension will likely be about ~635 pounds on the rear axle due to the leverage (based on 48" extension and a 142" wheelbase). So you will likely be a few hundred pounds over the rear axle and almost that much under on the front axle.
If you already have the truck, then you need to weigh it to see how the weights sit on the axle. Then figure that 100% of the camper and tongue weight will be sitting on the rear axle. If you don't have one, take one for a test drive across a scale (some won't charge if you don't need official weight).
Another thing to consider is the diesel will provide much better engine braking thanks to the exhaust brake. I've switched from a diesel to gasser and it's the biggest thing I miss. At highway speeds it isn't so much of a problem, but the back roads I travel when I'm going slow under 25mph, it's hard to get any engine braking from the gasser.
If I recall correctly, I think a lot of folks going with that large of a camper are opting for the F450 so they can stay under the weights while keeping the diesel.
