If it is a 12 volt fan, it may have a bad connection which would cause low voltage which might be the cause of a slow fan. You probably should check voltage at the switch that controls the fan and also at the fan itself. If voltage is good, here is a link to the fans that Trekwood shows that were likely installed in the 2021 rigs. You will not find 12 volt fans in any big box stores.
https://www.trekwood.com/parts-searc...23+%2F+Keyword