As mentioned, you do not need 4x4 just to get around in most of the western hemisphere. Some exceptions are if you do off road tracks, enjoy beach driving, or need to navigate deep snowy winters.
A smaller class C can do what you need. I would suggest a B or B+ personally. Under 20ft long would be ideal, shorter wheelbases (smaller turning circle) is ideal. A problem you will run into is maneuvering in cities, and tighter urban environments. Being able to do a U-turn on a standard width road (with shoulders) is amazingly useful. Especially when you don't know an area well.
Large Class Cs (over 21/22ft) especially those with large overhangs, can be a challenge to operate on tighter streets. When accessing campgrounds, or driving on steep mountain roads/streets, it is entirely possible to hang up your rear end on a dip or dip+turn.
Our current rig is just over 20ft long with a 12ft wheelbase. I think this is the max I would personally want for our usage. If you use mostly developed campgrounds, and don't need to negotiate narrow central/south american city streets often, longer would be okay.
You wan't a rig with excess weight capacity, and a robust body. Nothing as serious as a true expedition camper is needed. Running around right at max gross weight leaves you with minimal margin when you hit a bump or pothole at highway speeds. Those Mexican Topes can be brutal!
Another major consideration is shipping. If you are driving from central to south american, you must ship your vehicle across the Darian gap. The easiest and safest method is by shipping container (this is true for most shipping). If you want to use a container, your vehicle needs to fit! For a high cube container this is 2.585 meters high and 2.34 meters wide. Our van is 2.6 meters tall. With a set of tiny shipping wheels we can just barely fit in a high container. Being able to use a container saved us quite a bit of money and hassle when shipping to Australia.
If you want/need to get off the beaten path, a 2WD with good characteristics will go much farther than you think. Ground clearance, wheel articulation, approach/departure angles, tires, and finally driver skill are the most important things. Once you exhaust those, its time to go to 4x4. Honestly throwing a 8,000lb+ RV or class B rig down a nasty trail is not something you probably want to do anyways. For the truly remote or amazing places, you can just let your feet take you. Or, rent a dedicated 4x4 vehicle at the location, and use that.