Hi! Looking at a few used roadtreks, one is 92 roadtrek 210 independent. The other van is a 95 Dodge Ram 3500 V8 magnum. I've read a bit, but mostly I am still in the dark as to the quality of older...
Wow, that sounds like a good find and a great trip :).
As for storage, you might pile some of your stuff in the van, and some food and water, and take it to a truck scale for weighing. I found that in my van it's the weight, not the space, that limits how much I can carry. I still have some empty space. So you might not need more storage.
Edit: I found shop manuals (real ones, not just Chilton's) for my rig on ebay for super cheap. You might even see if you can get a set in Spanish.