I have basically the same coach but a different floorplan. I think that model year still has the side radiator, in ~2007 Winnie switched to a rear radiator to save some production costs. Side radiator is highly desirable - it's easier to access the engine for maintenance and belt changes, etc. Rear radiators need to be cleaned (steam cleaning is best) every xx miles (maybe every 12-20k miles?)
If you will be towing a heavy load in high and hot altitudes, the cooling pack on the engine is marginal - it is my opinion it is under capacity by maybe 20-25%. At 87,000 miles the head on my Cummins needed to be pulled for a valve job - the crankcase pressure was too high indicating the valve guides were work. A Cummins factory rep confirmed they had problems with that engine's head circa 2005-ish but crankcase pressure would go out of spec about 50k miles.
Build quality is pretty good on the Horizon/Vectra. Try to find a Winnebago Ultimate Freedom, I think the last year for them was about 2004. That unit is on a Spartan chassis and in my opinion is one of the finest Class A's Winnebago ever produced. They had the Cummins ISL rated at ~370 HP but the same 1200 ft. pounds of torque as later engines. (Ignore the increased HP of later model engines - it's the torque that gets you up the hill.)
My fuel costs average about $0.50 per mile - a thousand mile trip will cost you about 500 bucks in fuel, just wanted you to be aware this is not an inexpensive way to travel.