We've done all the above recommended routes and a couple more. The toll on the Mass Pike from the NY border to Rt 146 in Worcester was $3.25 in May. Getting an EZPass is just a normal thing to do when traveling in the northeast.
So I-80 or I-78 to I-287 to I-87 to I-90. You don't have to go all the way to I-495 on the Pike, you can get off at Rt 146 and pick up I-295 to I-95 to I-495. Looks out of the way, but I an assure you that bypassing the I-90/I-495 interchange is worth it.
Because that KOA is closer to Cape Cod than it is Boston, another route to consider is I-84 to I-691/CT66 to Middletown CT, then CT9 south to Old Lyme to pick up I-95. I-95 through Providence isn't usually jammed and you can pick up I-195 to I-495 north to exit 5.