You COULD use a single-axle tractor, but will probably have to stretch the frame a lot, which you probably would with a tandem axle setup, too. But a tandem truck would be better, mainly because there are TONS more of them, and they give you a choice of two wheelbases, depending on which axle position you choose to use, because most air suspensions are two separate setups, one per axle. You just remove the one suspension set you DON'T want, and use the rearmost(NON-power divider) axle in either case. With a tandem, you have a choice!
Though, on re-reading your post, being that you will be towing a fifth-wheel unit, you likely will NOT have to do any frame stretching in either a single- axle truck or tandem.
Does this help you?