We chickened out and put our tandem inside the Chrysler Town and Country minivan. Once we removed the back seat, which was a lot easier than I thought it would be, we had enough room for the tandem if we popped off the front weel. It was simply more relaxing and reassuring knowing the tandem was in the minivan with us rather than hanging off a trunk rack.
We still use the the trunk rack on my Dodge Stratus for short runs up to local parks. However, for long runs over 50 miles, I think we will stick with putting the tandem inside the minivan and just pay the extra gas cost. In fact, we never put the back seat back in and just might not do so for some time.
By the way, we had a wonderful time cycling around the Copper Country, but it did rain all day long when we planned to go between Eagle Harbor and Copper Harbor, so we have something to do on our next trip up north.
Thanks everyone for your advice.