I would think going over a crest would be the most critical issue with less than 3” clearance over the bed rails. I just measured my standard bed and the pin is 44” from the end of the rails/tailgate. I calculate a crest with a difference of about 7% would cause 5th wheel with 3” clearance to rub rails/tailgate. While that may seem extreme, 7% is well with the 10% max allowed on Illinois highways so I would suggest one be careful with just 3” vertical clearance on bed rails. There are locations that could result in damage to truck or rv.
A 6” clearance and would be more than 13% grade difference which does provide a 3% margin with normal design practices.
You are braver than me. I loved the simplicity and light weight of the Anderson hitch, but the engineer and farmer couldn’t see how it safely holding a 20k lb rv. I remember designing a project as a young engineer. The calcs said 1/4“ was adequate. During peer review a colleague suggested I up the thickness to 3/8” based on his experience. I went forward with 1/4” steel. Three months later, I tore it out and replaced it with 3/8” steel after it got bent.