...I understand what a Sous vide machine is and what it does...but is it all that difficult to keep a pot of water at a specific temperature of let's say 130 degrees over the course of two hours????
It is kind of hard to get it right, what happens is many stoves and cook tops jump ten degrees with a very miniscule rotation of the knob. My first attempt was with a thermostatically controlled induction cook-top, but there are easier ways. You must also put a towel at the bottom of the pot (which gets hotter than the water), to keep from exceeding the maximum temperature the seal on the plastic bags can tolerate. Right now the Anova Sous Vide unit is only $169 on E-Bay. It is the most economically priced unit, and is outstanding. Temperature accuracy and stability is less than 1 degree. The only downside, you must supply your own pot. As we speak, I am doing sous vide rosemary chicken breasts which will be browned over the raging heat from a kamado grill. I am still undecided on the glaze.
Something you could do also would be be preheat your oven to about 225 and also bring your water up to your set temp (135). Once your water on the stove is there, drop you food in its packages in and then pop that whole thing into the oven and turn your oven off. Wrap your pot with a towel or 2, and this should help maintain your set temp. You'll loose 3-4 degrees each hours likely this way, but you might be able to start on the slightly higher side and end up at a good place. Homebrewers use this method to maintain a steady-ish mash temp throughout an hour.