When we attend tandem rallies there is a lot of possibilities seen.
For us though
Ford E150 Chateau Van for keeping the stuff locked and out of sight.
Toyota Tacoma double cab long bed other times.
When we bought the Toyota, the van hauled a our longest tandem to the various dealerships. Salesman came out, we explained the bike had to fit. After that we (actually my heartless wife / stoker, beat them up on the last day of the year) worked the deal on the model we wanted.
PK