I-81 and I-88 are not toll roads. I-90 from I-88 to the NY/MA border also involves no tolls. (The portion between I-88 and I-87 is part of the thruway, but between those two points no toll is charged. If you get off at an intermediate exit there is a toll. East of I-87 it is not a toll road until you get into Massachusetts.)
I-84 is not a toll road, unless I'm completely confused, aside from the bridge over the Hudson which is a toll bridge (the toll is only charged eastbound).
I assume you're already aware, but propane is prohibited from the Boston-area tunnels, which means you cannot take I-93 through (well, under) the city, nor the easternmost bits of I-90...not that there would be any real reason to do so to get to your destination.