They're just not built well enough to survive the terrain.
I agree with Chris, it might be a one way trip to a very large and expensive camp fire... there's just not enough true structure to have it last the trip and still have it worth anything if you make it back home. The flexibility would rip it apart from the inside.
My advice would be a fully built van or a proper truck camper in a proper 4x4 pickup.
Without trying to sound sarcastic but am i missing something about traveling the roads through central and south america, are they ox and wagon trails? I know that if you want an offroad adventure or technical 4wd conditions that you can find it there depending on route. But i'm just going off what the OP was saying about setting the stage for his trip,
" dirt/unimproved roads, some beaches to surf/camp, no hardcore wheeling" and to say a small Class C can't do that is way off. There is no question that there is a huge difference between brands/build quality from stick frame to aluminum framed to full fibreglass shell as well as materials used on the interior and how cabinets are mounted to walls and I think that is better to be addressed than just writing them off. Washboard roads will shake ANYTHING apart. People have travel through central and south america in all sorts or cars, trucks, vans and campers ranging from old and new and i don't believe that you need a fully built expedition vehicle to travel their roads and to have a great experience.
I know a lot of people, many personally who would disagree with you including myself. Have you tried looking at different vans in person? Have you tried van living for a little while? Have you talked first-hand with any van dwellers? Don't dismiss the size because function is a rather amazing thing. It does take a different mindset to get used to living in a smaller space but completely doable and enjoyable if setup for your specific needs.
The only thing we can agree with on this forum is how awesome it is to get out and have an adventure. How to get there has and always will be debated and will remain a personal preference. I to have done the sportsmobile pop top, then a fixed high top caper van and for the sanity of my wife and kids and to ensure that they'd continue to experience exploring with me i sold those and moved to a mini class c and its the best thing for OUR family that i've ever done, and it hasn't limited us at all in what we do and where we go, which will be different than a guy that has a wife that is thrilled to be in a van or landcruiser with a roof top tent. We go all the same places we choose to take our 4x4 vans. Again, where some people choose to take their vans may be different than others. You look at the needs/wants of your family then try that route, build it to suit your needs and go places that you know that vehicle will take you, worst case you turn around. Its no different than a van can't fit or go where a jeep can, but the van will go 70% of the places the jeep will, so if a jeep doesn't work for you, then get a van and just don't go that 30% of places, which may not be where you wanted to go anyway. If a modified C will go 80% of a Van, then just don't go that 20% of hard core places, most van owners don't take their vans rock crawling, or technical offroad trails.
My C goes 100% of the places i want to go and explore, and that's not all highway which you may think. Just my personal opinion.