The Ford E450 has a fuse and a relay to supply 12 bolts to a towed vehicle's battery. Look in your Owner's Guide in the section "Roadside Emergencies". The fuse and relay are in the Power Distribution Box under the hood. Look for "Trailer Tow Battery Charge". Be aware that the relay is only energized when the ignition switch is in the run position. This is so your towed vehicle does not drain the chassis battery.
If you don't have the Owner's Guide, you can download it from a Ford web site. I don't have it handy but can get it if you need it.
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