We are in the process of replaceing our GW Profile Microwave and the install instructions say you need to remove the fans in the microwave and reinstall them reversed and rotated in order to get the vent to exit out the back. The new one is shipped to recirculate to the inside and external venting needs to be reconfigured. The install instructions are too big to attach but if you want them go here GE Microwaves - Microwave oven, Microwave convection oven from GE Appliances
Search for JVM1790 with Manuals in the pull down box, then select install instructions.
We have the JVM1790 look for Install Instructions.
Yours may not have been installed correctly (It happens on all coaches)
It requires you to remove the oven so be sure it's wrong first. I found that to be no diferant than removing one in the kitchen.
Our range hood and fan is built into the microwave.