Wow! Your prius weighs THAT much?!? Will you be using a dolly ( about 200 lbs) or a dual axle 16' trailer ( between 1500 and 2000 lbs) . I can't imagine the prius weighing more than 2500, so Mabye that's where you came up with a tow load of 4500??
Anyway, my bus ( avitar pic) is a 2000 gmc savana ( chevy express clone). It weighs 6500 lbs, empty. When I moved from Ny to Ala, I towed my backhoe (Kubota diesel, about 4500 lbs) on a 18' dual axle trailer ( the trailer weighed 3000 lbs empty according to the registration) for a total of 8000 lbs, plus about 2000 lbs worth of assorted tools and "stuff" for a total towed weight of 10000 lbs or 5 tons. I towed it at 60ish mph all through the , poconos of Pa, appalachins of Md,Va and Tn, and smokies of southern Tn and N Alabama. I averaged about 10 mpg in the hills and 11.5-13.5 the rest of the time. The bus has the basic Gm 350 cu in engine with factory 4 speed automatic ( probably the same eng/trans set up as the MH you are looking at). I did not have anything special done to it to set it up for towing except adding a 12k class 5 hitch and electric brake. It came factory with a small trans cooler, but if I were to tow all the time, I'd upgrade to a larger one. And as always, common sense towing suggestions/rules apply. I would not care to tow that much weight ALL THE TIME.
Could you do it ? Sure, no problem, but double check the manufacturers load and towing recommendations to be sure. And id defiantly look at a true dolly vs a trailer to save the almost ton of weight a trailer will add.