Being in Central American countries from Mexico to Panama via cruise ship shore excursions and similar, I would think twice about driving from the USA to Panama. I would be concerned for our safety. Americans in a motor home with USA plates, will stick out like a ripe piece of fruit on the vine. There is a steady stream of illegal migrants walking north on major roads. Most people are good, but there are some really bad apples too. People in hard times can get desperate.
It's not much different as buying a brand new Corvette convertible, put the top down, have $100 bills sticking out from my hat, crank up the stereo, and drive through the worst of neighborhoods in Chicago in the summertime. It's the perfect way to commit suicide without voiding my life insurance.
Also, like others here mentioned, repairs will surface often, resources for replacement parts will be few, and reputable service centers less yet. Pending the type of repair and location, you could find yourself stranded for months.
As much as I would love to RV all the way to the southern tip of South America, I will never take our motor home south of the border.
Call me paranoid.