You are correct. They do not come with a filler tube.
Did you buy the top fill, or side fill tank?
Top fill you have to go through the bed, side fill you can do what kaycee above did.
For top fill most people use a boat/marine/motorcycle style filler cap.
Here is a great site to get ideas.
https://www.fillernecksupply.com/
Mine is the side fill, same as kaycee's. I used a mid 90's Chevy S10 setup including the door hinge assembly, plastic filler tube pocket, and the top 6 1/2" of the filler tube. Bought for $15 at a junk yard. The rest I cobbled together from the auto parts store, and Lowes. The cap is a Permacap which has a trap door top just like the new cars and trucks.
I did have to notch the left rear frame rail, trim the bottom of the left bedside(red arrow), and trim the bed support sill that runs over the tank as the tank top is above the frame when installed.
I also converted to LED side marker lights so there is no issue with the light being so close to the filler.
It fills pretty good, but you have to go at about half speed on the nozzle flow, or it backs up.
Here are some pics of the install.
There are tons of posts on this forum, just search for rear tank conversion and you will find plenty to look at.