I have this same question/problem. Looking at pictures, it looks like the body lines between these two styles are essentially the same. The tailgate has some body lines that are different, and the Ram emblem seems larger, but they seem to be the same overall.
I will be taking measurements between the two and if you know something I should check that I may not think of, I would love a heads up. I am mainly planning to check placement of mounting holes, etc. And also distance from bottom of the bed to the body lines.
In my case I am looking to put a 2020 2500 8 foot bed on a 2010 1500 truck. So far this looks compatible…
As was mentioned above ..
2500 & 3500 trucks are Generation 4 Rams, years 2010 to now
also 1500 Ram 2009 -2018 are also 4th Generation
in 2019 to now the Rams with the CLASSIC designation are also 4th Generation
The 5th Generation Rams may or may not have 8 foot beds ??
The tailgate on a 5th Gen has a larger emblem & the top of the tailgate may have a different shape, the old 4th Gen captured water
So... I got the beds swapped around. I now have the 2020 2500 8ft bed on my 2010 ram 1500. Looks perfect. Body lines match up, all is good. Sort of. The gas filler is different. On my 2010 the plastic housing for the filler neck is just sort of clipped to the bed. On the 2020 it looks like there are three screws that hold it on. The filler was not included with the bed so I am doing some research. I found a part number for the filler housing, but I suspect I may need the top part of the filler tube as well. Still not sure. I looked on my 2017 Ram 1500 and it has the screw in style housing, probably the same as the 2020 bed. The filler neck looks to be riveted to the housing, but mine has no provisions for that. On the 2010, there is a little bracket on the filler neck that gets a single screw to the bed for support. i may have to hope for a little help from the dealer parts people... Unless someone here knows what I need here.
The new bed came with really nice taillights but unfortunately, they seem to want separate turn signal and brake wiring. I don't have a diagram for the 2010 rear lighting but I suspect that they never did separate turn and brake lights so the wiring is likely not right to allow me to use these super nice taillights. The third lamp housing is clear and I assume it is supposed to have amber bulbs for turn signals. I considered adding a third socket , wiring it to the existing brake/turn signal wires and then using a red bulb for that, because it is a clear housing. Kind of a hack, but possible...
Also, as expected, and found mentioned all over, the tailgate is electric and my truck is not configured to do that. The harness that connects to both latch solenoids was still with the bed, but I am having trouble finding the connector for the input that fires both solenoids. It is a small oval two pin male connector. It looks like it is the same connector that would plug into the electric tailgate switch, but the harnesses I have do not have any connectors like that. They must have kept those harnesses. I am not trying to be too clever here. I would be happy for now with a switch somewhere that would send 12 volts to the solenoids. Just something that will reliably open the tailgate.
I am considering looking for a way to wire in a latching relay to switch the power to the "tailgate switch" so that when the doors are locked there is no power to that switch. Then when the power locks unlock, the latching relay could enable power to my switch again, so I can unlock it. I haven't worked through that yet, but it feels possible. If I get that working, I can detail it on here if anyone is interested.
But my big issue for now is the connector to the latch solenoids wiring harness. I need the female to plug into the male that is in that harness. Is it a standard connector that I could just buy or is it Chrysler specific? Does anyone know?
I am not sure about the truck the bed came from, but my 2010 and 2017 are both gas and have the two different styles of filler. It feels like the early ones, up to about 2012 have a lot of differences from the later trucks. Like they fixed everything they got wrong originally...