Minimum clearance?

There's no magical number but 6" seems to get thrown around as a good minimum from what I've seen. It just depends where you travel to. I'm at about 7.5 and have been on some pretty hairball uneven logging roads and have never hit.

There has been a lot of change in trucks and 5th wheels over the years. Presumably, the 5th wheels have changed trying to catch up with the ever-rising trucks. My newer Ram truck and newer 5th wheel worked out great despite some fears that I wouldn't have enough clearance. The worst scenario seems to be an old 5th with a newer truck.

6" is kind of the minimum rule of thumb but 8-9" is better...particularly if you will be on rough roads or uneven camp sites.

Dilemma is you still need to keep the trailer close to level and newer trucks sit higher than older trucks.

Pull the fw onto enough boards to make it level. Then measure the rail clearance. Just say it's 3 inches and you want 7, you then know that you need to lower pin and or raise the hitch 4 in and lower the trailer suspension 4 in plus the thickness of the lumber under the tires.
There are a couple tricks that will raise the rv (lower the suspension)) a small amount but if you're that close to the rail you are more likely looking at an axle flip or sub frame.

If you are looking for lift of the RV it may also be the time to look at things like 16 inch tires vs 15. It doesn't raise the rig much but it can help.

I have right at 6” and coming out of an uneven driveway at a a gas station I almost touched the bedrails.

Is the measurement from the rail to the underside of the nose or the bottom of the tongue?

I'm at 6.5" clearance at the lowest setting for the ball on my AUH. I raised it to the next setting and I was a bit nose high. It didn't pull as smooth as it did at the lower setting. If your hitch is easily adjustable, try different heights to get your 5er level. It will pull so much smoother if its level.

Super_Dave wrote:
Is the measurement from the rail to the underside of the nose or the bottom of the tongue?

bottom of trailer overhang to bed rail

My trailer is sitting on the ground in auto level mode. The pin is much lower than the liowest ball setting of the AUH.

When you add pin wt to hitch/truck, it should make the truck squat a little, which will help maintain a level tow. A slight nose-high is normally not an issue. You may have room to raise the pin if needed.

Jerry