Something to try. Let it sit for a couple of days then try to get a really small diameter vinyl tube down the fill tube. If there's a check valve ball in there you might be able to get past it, might not.
If you can get a tube in and down to the bottom of the tank you should be able to pump out a few gallons and hopefully get out any water. Any of those gasoline 'antifreeze' and other things that are supposed to take care of water in gasoline can only do anything with a very small amount of water.
What happens when water gets into an ethanol/gasoline mix is the water makes the ethanol separate. It essentially 'washes' the ethanol out of the gasoline. The ethanol/water combo will settle to the bottom of the tank. In a vehicle that's not flex-fuel the lower energy density of the alcohol, combined with the water, will make it run like crap because the computer cannot compensate by injecting more.
Of course crap like getting bad gas or the fuel pump dying most often happens when you fill the tank up to the top. 100% of the electric in-tank pump failures I've had in GM, MoPar, and Ford have happened immediately following a complete fill.
Where to drain 35 gallons of gas? Buy or borrow seven 5 gallon gas cans. Or see if a salvage yard has a 40 gallon rear tank from a 'square body' GM truck from the 70's and 80's to use as temporary storage.
If it does turn out to be an excessive amount of water, you should be able to let it settle then carefully pump the water out of the bottom of whatever you decant the truck's tank into. A pump like this works and won't blow you up.
https://www.amazon.com/Ketofa-Siphon-Liquid-Gasoline-Plastic/dp/B004107292
I used one of those to pump a veritable lake of water out of the bottom of a tank in an old Nissan, which had a removable plate in the bottom of its trunk, and another on in the top of the tank. So nice to be able to pop open a hole around 8" diameter and see the pond under the gasoline. Sucked out about a gallon of water, re-assembled, installed 8 new spark plugs in the 4-banger (yeah, like an old "twin ignition" setup from the 1930's) and it started right up.
Something to do once you get the suspect gasoline out is to put a little into a shallow container like a tuna fish can then light it. You should get a *much* smaller scale version of this kind of flame. Of course do that *far away* from anything that'll go *foomp*.
I got some really bad gas in a 1998 Sebring convertible. Ruined the expensive pump, of course after a full fill. That fuel was extremely difficult to burn. Once I got it lit the flame was very low, not roiling at all, extremely smoky. I suspect someone screwed up at the tank farm and got some transmix into the tanker. Might've been mixed with diesel or some other petroleum product. Of course the gas station wouldn't pay for the damage to my car, or any of the other vehicles that "fuel" ruined the pumps in.
Last vehicle I had a pump die in was a 2004 Dakota crew cab. Easiest way to fix it was to remove the bed, then drop the tank. Having the bed off made it much easier to get at the hoses under the back of the cab.
Installing a new pump would be a very good idea, especially if you find a lot of water in the tank and if you've 100K plus miles on the truck. You don't want to have to be dropping the tank again.
