will a group 24 and a 50watt panel be enough for 2 week trip

I haven't been there but from what I understand most or all of the NPS campgrounds in Yellowstone have showers. That should be a savings for you. I assume that by boondocking you mean those campgrounds. There is no just pulling off and camping in National Parks, not allowed. From what you describe as your kind of camping, adding a battery and going to 200w of solar would make you very independent.