Freightliner cascadia fuel tank cap won’t close

Does the lock turn ?

 

Looks like the the tank flange is bent down on one side.
It also looks like when you turn the cap it bottoms out on the tank before it locks in completely.
Is it possible to remove the cap and find a piece of round hardwood a little smaller then the tank hole and then carefully slide it in there and hook a come along to it in the opposite direction a give it a few clicks to bend the tank neck up straight again?

 

Have you checked and seen if the locking mechanism is working on the cap?
Have you tried it with the key in and turned backwards?

 

If you stick something in the tank hole that is relatively soft but also ridged (like a round piece of hardwood from a limb of a tree and pull it in the opposite direction of where it is bent down it will bend it back up.
The only caveat is you would need a round piece of wood that is just slightly smaller than the diameter of the fuel tank neck.
If you use to small of a diameter piece of wood it could potentially oval the fuel neck when it bends back up.

 

Make believe the piece of rolled up cardboard stuck in my fuel tank neck is really a piece of round hardwood.
You then pull on it in the opposite direction of where it is bent down and it will bend it back up so the cap will clear the fuel tank when it’s fully turned and seated.

 

I know you didn't really try cardboard...?

Both tanks *should* be slightly tipped to create 'outage' space. Outage is the allowance (10%) in the tank for expansion of the fuel when it gets hot. It looks like all your outage allowance is in one tank.