Dual tank fill-up to top

That third hose is a vent hose for something. Most likely the evap canister. That cap is there to keep dirt, water and critters out, but should allow it to breathe. Grab it with your fingers. It should have a loose feel.

Little quick googling shows some similar issues as yours. Fwiw there’s probably a better way to figure it out than this forum.

Links with adds is a lame way to post pics, but whatever.
The “3rd line” is your diff vent likely. Nothing to do with the fuel tanks. The smaller of the 2 fuel hoses is the filler vent lines.
And nothing looks “wrong.”

PA12DRVR wrote:

ognend wrote:

eHoefler wrote:
The fill necks are too low. The after market bed manufacture has them too low.

Assuming this is correct - what is the fix? Thanks!

Had a sort-of-kinda-like problem on my recently sold dump truck. First thing to look for is whether the fuel fill spot ("fuel cap") is higher than the tank(s). Second, if the "fuel cap" is indeed higher than the tank(s), does the fill pipe run between the cap and the tank in such a way as to allow fuel to gravity flow down or is there either an extended run of level fill pipe or does the fill pipe even run "uphill" from the fuel cap before dropping down into the tank.

On my truck, it was the latter situation: there was a slight, very slight, uphill run between the cap and the first bracket on the hose. For whatever reason, it would fill fine until the tank was 3/4 full, then I couldn't get any more fuel in the tank...it would just come back out. I lengthened the bracket, had a local shop shorten and straighten the fill pipe (so it ran entirely downhill between the cap and tank) and the problem was resolved.

Another option of course is to put the fuel cap at a higher location on the bed, but that's probably difficult with the aftermarket bed.

Interesting. Well, I took photos and I found two things. Only the back tank has a 3rd hose that is capped (is that the vent hose)? Otherwise, yeah, there seems to be a slight drop but I cannot tell if it is sufficient or not. I am posting links to the photos.

Back tankhttps://pasteboard.co/T5MJxUeXoa2j.jpg
Front tankhttps://pasteboard.co/QrLKpvlaH9LL.jpg
Back tank drophttps://pasteboard.co/iEd5zn8f4OxH.jpg
Front tankhttps://pasteboard.co/EvLqBWXRm8cF.jpg

Hopefully you’re still under some sort of warranty from the up fitter if it’s not a GM issue.
Waiting 3 years to address it may leave you holding the bag if it’s an upfitter issue or past the 3 year mfg bumper to bumper.
I’m guessing not many, if any, here, have that setup as it is only offered with C&C gassers I believe. Which are relatively rare to start with.
If it’s not simply an error in mounting the fuel fillers, then idk.
According to the OM it says if you run out of gas fill the front tank first, so I’d surmise that the engine always pulls off the front tank and the rear pump is a transfer pump triggered by level sensors to either keep the front tank full until the rear tank is exhausted or to trigger the transfer pump periodically to re-fill the front tank partially (since the rear tank is bigger).

You say sometimes it works but realistically you’ve filled them less 10x or less based on the miles.
Do you have any data to help you figure out that sequence and also how low in which tank you were when refilling? You could theoretically use 40 gallons of gas and the front tank still be plum full.
And yes of course you can fill the front tank (if it needs gas). But depending how the truck calculates total fuel/miles left, you could be around “1/4 tank” on the gauge and be at 3/4 full or more in the little front tank.
Since you’ve procrastinated 3 years, now is the time to maybe learn how the fuel transfers to eliminate user error or ignorance from the equation to pin point an actual issue if any.
Easy test, run that sucker almost empty until you know you’ve pulled a bunch from the front tank and then see what happens when you fill up.
I presume the dual tank models only have one gas gauge and the mpg and dte info takes both tanks into account?
Both my newer chebbys in the last few years have been spot on with dte and fuel gauge readings (leaving about 2 gal in the tank when dte miles goes to zero and actual tank capacity at rated cap when the truck runs out of gas (once, lol)). I’d think yours is also reliable. I’d suspect your low fuel light should come somewhere between 5-10 gal left in front tank or total in the system.

ognend wrote:

eHoefler wrote:
The fill necks are too low. The after market bed manufacture has them too low.

Assuming this is correct - what is the fix? Thanks!

Had a sort-of-kinda-like problem on my recently sold dump truck. First thing to look for is whether the fuel fill spot ("fuel cap") is higher than the tank(s). Second, if the "fuel cap" is indeed higher than the tank(s), does the fill pipe run between the cap and the tank in such a way as to allow fuel to gravity flow down or is there either an extended run of level fill pipe or does the fill pipe even run "uphill" from the fuel cap before dropping down into the tank.

On my truck, it was the latter situation: there was a slight, very slight, uphill run between the cap and the first bracket on the hose. For whatever reason, it would fill fine until the tank was 3/4 full, then I couldn't get any more fuel in the tank...it would just come back out. I lengthened the bracket, had a local shop shorten and straighten the fill pipe (so it ran entirely downhill between the cap and tank) and the problem was resolved.

Another option of course is to put the fuel cap at a higher location on the bed, but that's probably difficult with the aftermarket bed.

eHoefler wrote:
The fill necks are too low. The after market bed manufacture has them too low.

Assuming this is correct - what is the fix? Thanks!

The fill necks are too low. The after market bed manufacture has them too low.

valhalla360 wrote:
Why would they put a fuel fill on the front tank if you weren't supposed to use it?

As others have said, sounds wrong.

I understand and I agree about the "why".

Why would they put a fuel fill on the front tank if you weren't supposed to use it?

As others have said, sounds wrong.