You didn't buy the original size belt, could be it is wider or narrower, or the most likely, not have the correct number of ribs? When counting, you count ribs on the belt, grooves on the pulleys.
I suggest buying the correct belt. Assuming you have the original size pulleys and idlers, run a length of string around all of them just like the belt installation should be; measure total length of that string to obtain proper belt length. With that measurement and correct number of belt ribs (see above) go to your nearest NAPA store and ask for help buying the correct belt.
Example, when I replaced my serpentine belt, NAPA matched it to a belt for a Freightliner MDT.
update; I just saw laj's reply, and most likely that may be the cause.
2000 Winnebago Ultimate Freedom USQ40JD
, ISC 8.3 Cummins 350, Spartan MM Chassis. USA 1SG, retired;PPA,Good Sam Life member."We the people are the rightful masters of both the Congress and the Courts - not to overthrow the Constitution but to overthrow men who pervert theConstitution. "Abraham Lincoln"