Need advice if safe for my pickup to pull a heavy 5th wheel

For all but the smallest 5ers, the trailer will weigh more than the truck. That's OK.

While the pin weight is still part of the trailer from a functional point of view, yes it helps plant the trucks rear axle solidly in place. Where a light duty truck can run into problems is when you overload the rear axle. The springs & tires start flexing more than they were designed for... resulting in a wobbly unstable ride. You also have the risk of those and other parts failing as they are loaded beyond their ratings.