Do a search on "wax valve" on this forum--there is literally hours of reading on this subject. The "short version" is that 2003 and earlier Alpines were designed with a mechanical wax valve to control the engine fan. Newer Alpines use an electronic fan interface. On the 03 and earlier, the valve opens and closes based purely on the diesel engine's need for cooling--there is no interface with the dash A/C. Reviewing the specs on the wax valve suggests that it maintains limited RPMs but the fan on my 03 will not turn until the engine calls for cooling via warm coolant in the radiator.
Accordingly, the only air flow across the dash A/C condenser on your radiator stack is limited to ambient flow at highway speed or if the engine is warm enough to open the wax valve and activate the fan. After market solutions include: 1] an electric solenoid place inline to the fan motor fluid controller line that is linked to the dash A/C controls; 2] if enough room for it, adding an electric fan to the outside of the stack--again, linked to the dash A/C; or 3][as I did] move the condenser forward to clean air and add electric fans, and again, link it to the dash A/C.
You can PM me on this forum if you would like to discuss this issue in detail, and/or to discuss possible solutions.....
2003 40' MDTS
Garden Ridge, Texas