My truck is a cab & chassis so I'm not sure how much of a difference that makes, but...

The truck came with a huge utility box on it and the part the fuel cap screws into was pretty much just set into the hole in the box meant for it - it has a triangular flange with 3 tiny screw holes. That part has a tube coming off the side to connect to the vent on the tank.

Now I have a very wide, low flatbed on it and it has to sit practically horizontal, which just is not working for me, but I don't know what else to do. The only thing that seems like it'll work is putting the filler through the bed 2 feet in from the side, but that would really restrict how I can use the bed. Basically I'm looking for ideas on how the heck I can fill my tank better... right now I can't use big truck stop type pumps at all, and even the diesel nozzles at gas stations I have to fill it real slow to keep it from shooting out the filler. In the overall pic of the bed you can barely see the fuel cap, it's tucked behind the outer rail. I had the filler welded in place but it was at even more of a horizontal angle, this way it was easier to fill (slightly).

I'm also wondering if there's a PN for the thick rubber thing that keeps the base of the filler neck in the tank. I don't think mine leaks but it's obviously been flexed so much (and is old) that it's cracking. Ignore the crappy vent tube, it was an emergency repair.

 

The rubber gromet for the fill pipe to tank can be had from your local
dodge dealer. I got one last fall. I don't remember the exact price, but it wasn't bad.

 

What about poking a hole in the side of the flatbed then putting some type of "s" shaped pipe to connect to the fill pipe? As long as it's higher than the tank gravity will do the work for you! And I'd think about making that vent quite a bit bigger, that will help get fuel in there, I know I was supposed to Ignor that part, but you brought so much attention to it...LOL
Tell me if I'm way off base on what you are trying to accomplish with this.

 

The vent tube is supposed to be 5/8" hose (it's hard to find 5/8" fuel hose...), but it wasn't filling well before I "fixed" it either.

I'm not sure what you mean about an S-shaped thing, I have thought about notching out part of the bed to get the filler up higher, but I'm not sure how much it'd help since it'd only be going up like 2 inches.

I also thought about cutting down the metal part of the filler that's connected to that gasket in the tank so I could just crawl under the bed and fill the tank directly, but that's not a very good solution

 

on a few trucks at work that have utility bodies swapped out to a flatbed they are almost nearly impossible to fill quickly. One that does work well seems to have a custom setup, its the rollback. The fill holse is welded where the mud flapp support which is welded to the frame about 6" from the underside of the deck. This one i can fill up the fastes being that there is only 1" or so of hose and its still above the tank, with enough clearance to get the pump handle in there nicely. The only down side is when its snowing, there is a lot of buildup down there so you have to boot the brown/black snow away to get to the diesel cap.

I hope i was clear enough, i'll see if i can't get a picture for you tuesday....

 

Get a good 6" high piece of wood with a wedge cut in it. When you fill drive up on the wood on the fill side should help out a lot when filling.

 

I actually forgot about that, there is a 1' difference in elevation between the fill tanks and the driveway, i usually drive up on that been doing it for the last 4 years and i forgot all about it!! It does get the last few CCs of air out of the tank and replaced with fuel.
Do it on a cold morning and you can't drive fast enough to use the fuel up before it expands and overflows out of the tank.

 

You might try C notching the side frame rail and put the actual filler layer with a straight shot into the main hose and then go back to the fuel tank as close to where the event comes out if you can get enlarged the event to 3/4" and then run it to the neck or if you have a headache rack run forward an out and out. If you do that make sure you put some kind of a filter or cap To keep raining out.
The idea is to get a straight shot from the filler to the tank, you may have to hold the nozzle but it should take fuel a good rate.

 

Here is a shot of the filler pipe on my parts truck, it had a flatbed on it.

 

That's pretty similar to mine... I wonder if running the filler forward diagonally so it's longer would help any.

 

The vent is probably the biggest part of your problem. There's also a rollover valve/vent on top of the pickup/sending unit as well, that might be contributing to your filling problem, if it's clogged. But either and/or way, venting is most likely your primary issue.