Alright, so your E-450 is acting like it’s got a dead battery, even with a brand-new one — but runs fine once you jump it with the RV batteries? That dim battery light before starting is a big clue, though.
Here’s what I’d check, in this order:
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Battery cables & grounds – Just because they’re tight doesn’t mean they’re clean. A corroded or internally frayed cable will drop voltage under load. Check both battery terminals, the ground strap to the chassis, and especially the engine block ground. Clean them down to bare metal and tighten everything up good.
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Positive cable to starter – Could be corroded inside the insulation or loose at the starter solenoid. If the starter isn’t getting full juice, it’ll act like a dead battery.
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Starter drawing too much current – If it’s the original starter and you’re running 20-year-old hardware, the starter motor might be tired and pulling way more amps than it should — which makes the new battery look weak.
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Ignition switch – That dim battery light when you turn the key could point to a bad contact in the switch. Not super common, but not rare either on older Fords.
You’ve already proven it’s not the battery by jumping it. So now you’re hunting voltage loss under load, which is either cables, grounds, or the starter itself. It might be worth grabbing a multimeter and check voltage at the starter while someone turns the key. If it drops well below 10V, you’ve got a bad cable or starter dragging it down.
Of course, we can't advise you much on the troubleshooting given the limited amount of information. But when you feel like it, you can always reach out to us with more details on your E-450 model and we will assist you further ✌🏻
