The main difference is the material used with these two type of frames, that and of course the design is different. Of course then the person who's welding it together. Notice the material used on this GN trailer.
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This isn't the best photo of a 5er RV frame but you can see the difference in the pin box area. This is why most RV 5er manufactures will void the warranty.
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The frame material and design are a apple vs orange comparison.
Having said that, I've seen plenty of folks do it but they seem to be mostly smaller 5er's with less weight.
Good luck whatever you do
On edit maybe IB516 can share a photo or two of how some are put together :W