I doubt the float is bad, most likely scenario is the pick up foot has broken off. Common in these trucks and it usually shows up at about 1/4 tank.
The repair involves removing the tank, and adding diesel fuel rated hose. You make it longer than needed so it lays on the bottom of the tank, but cut the end like a fish's mouth so it can't get stuck and create a suction.
Do you commonly fill both at the same time? When my pickup feet failed they both failed around the same time, both were gummy as if the rubber had suffered attack by some kind of solvent. It happened to a lot of people around the same time, I speculated that they changed additives around then. No proof though
The sending unit strainer and resister for the gauge both seem to fail at about 20 years. Sometimes the strainer is brittle and breaks off and sometimes it turns to mush(could be additives or bio fuel). New strainers can be had allover. LMC has them for $40. But if you are in there its best to do the whole sender for both tanks. Diesel senders are impossible to find but 5.0 and 7.5 senders can be had. Both of these engines are injected so they are 2 line systems. The 7.5 comes without a pump so you have to rig a pickup/strainer. The 5.0 only come with pump, pickup and strainer as an assembly. These pumps are pass thru pumps so the mechanical lift pump will continue to work, no need to hook up the electric pump. Both will have a 4 pin connector, you can get a 4 pin pigtail for $5. Make sure you get the Gas sender for your year truck as they changed the resistance a few times. And I also agree removing the bed is the way to go, even 5th wheel and goose necks installed.