Extensions for diesel fill up?

Not to be rude, but no gas, diesel.
You can buy a flexible funnel of any length if you want, but I have never had any trouble putting it in the hole.

 

The only place that would be a benifit to it is small "car" gas station where the diesel pump is only on one side and not long enough to reach the passenger side tanks.
When I use to do local, I would had to fill one tank, then drop the hose and spin the truck around to fill the other side.

 

A few trucks have the fuel openings in such a place as to make it hard to get the nozzle to settle in in a way that it tays securely without possibly "falling out". These are rare. But any extender would have to "screw in" to lock and this would only put it in closer proximity to whatever was in the way in the first place making it then impossible to get the nozzle in.

And you can't add an extension to the nozzle else the auto cut-off would be rendered ineffective and you'd end up with over-flows.

 

On some trucks I have seen the tanks roll up from off road work. Just loosen the tank strap and pull the tank back into place then tighten again

Sometimes need to replace the rubber between the tank and strap

 

If the nozzle doesn't seem to hold in the right place while pumping diesel into the tanks, many drivers, including me have extra bungee cords nearby to secure the filler hose handle in a safe position while fueling.