If the tree is small enough for a sawzall to cut through, it's probably small enough to drive over too. As in, it's not even worth stopping for. If the tree is big enough to be a real obstruction, it's probably also one that'll take a lot of chain sawing to clear. And, as awesome as M18's are, I find it hard to think you'd have enough battery power to effectively clear something that big. Also, and maybe it's just me but every single time I've come across a fallen tree, it's been dead for a very long time and that takes a lot more effort to cut through. I've been doing a LOT of cutting of old dead trees with my gas powered 20" Husqvarna and can also tell you that they'll wear out chains in no time at all.
That said, I typically carry a sawzall on harder trips and mainly to work on Jeeps.