Our tandem takes up 81" of width from the rear tire to the left front handlebar (wheel turned sideways). That protrudes from both sides of our VW Eurovan Camper (which can also accommodate our tandem upright inside if we remove the middle and rear seats). The rack is a sturdy Yakima rack slid into a 1.25" receiver hitch.

Our Eurovan is 72.4" wide unless you count the mirrors, in which case it's 85.6" wide--a few inches wider than the tandem. We've used this frequently without problems. A previous receiver hitch-based bike rack with swiveling arms once dropped its arms, dumping our tandem on the highway. This one has fixed welded arms. If anyone wants to go this route I advise against anything with folding arms or with just one mount, rather than two separated arms. I think a tandem is too long to be stable with one of those unitary mounts.

We considered all sorts of options, but basically we can't put a bike on the roof--the roof pops up for camping--and even if we could, it's a bear to get a tandem on/off a roof mount, even with the modern swiveling types. Our rack makes mounting/unmounting a quick one-person job.