Fan on switch

[tebreuer]

I have a 1999 AS LY 355XL with a 11000 btu Dometic A/C in the back. When running this A/C it works fine with the fan set to auto. With the front 13500 btu unit, when the fan is set to "on" it runs constantly and the compressor cycles. With the rear unit the fan will not run constantly in the "on" position, but the auto position works fine. Is this normal, or fixable if it isn't? I don't particularly want to buy the $300 thermostat system just to get the fan to run constantly.