I'm planning a long trip - ride only 'local' buses only from the US to Tierra del Fuego. The way I travel, fifteen countries will take me around 4 years!
I have traveled the 'chicken hauler' route through Latin America before, and one thing is for sure, if it isn't secured in your lap, - its gone! I might skip the laptop - camera - iPhone type baggage for that reason as well, although I will need something to pay for the trip with Bitcoins, which requires at least the code and a smart phone - still working on this, because I don't want to carry much cash.
The Rolf Potts 'No Bag' site is a great idea, but too extreme for 4 years. If I add the right bag, however, one that can remain in my lap on the bus, be carried through markets during stops, checked into motels along the way, it might work.
The only clothes I would take are what can be comfortably worn, washed and dried every day (or so). So the bag would carry: underwear, socks, swimsuit, water filtration, towel, sketchbook, (maybe a spare shirt) food, and the sundry stuff that seems to accumulate. Of course anything can be purchased or discarded along the way.
My favorite of the moment is the Super Ego - more volume for less space, and can be used as a pillow.