I don't recall having any issue with traffic on the ACA Northern Tier route through Maine. However, I road this section to the ME/NH border in early May. In contrast, auto and RV traffic along the coastal portion will be much heavier in the summer.
If you want to stay off of US 1, you easily can. Maine has an abundance of rural roads that are great to ride. If you are willing to purchase the Delorme Maine Atlas and Gazetteer, you can use that to plan a ride that avoids US 1 easily, which is what I did when I took detours off of the ACA route. A route off of US 1 and ACA's Northern Tier certainly won't be the most direct route, but could be more pleasant, in my opinion.
Although, if you are riding from Maine to Iowa, then it may not be worth purchasing a whole giant atlas for a state you will most likely be spending less than a week in. You could plan an alternate Maine route when you get there by stopping in Rockland's (or any Maine) public library and looking at their Maine atlas. You can plan an alternative to the ACA route pretty easily and then just photocopy the appropriate atlas pages to take with you.