NEW QUESTION, was: Small lightweight TC for '06 2500 megacab

The 3500 SRW trucks don't actually have any heavier duty axles under them than your 2500 does.
In the real world, you gain nothing, all the factories do on a 3-series is put a little more spring in them. They do not have heavier duty axles. Your truck is entirely capable of hauling whatever camper fits on it, it may want some springs or airbags.
However, a long bed opens up a lot better options for camper hauling.