To find the steel rafters, I advise to get a pair of magnets out from an old computer hard drive. They are extremely strong. They should be able to find the steel beam easily. These are examples of computer hard drive magnets.


Ask the PC factory this question: Is the steel rafter a box beam, an "L" beam or a "T" beam. If a "T", you won't want to hit the center. If an "L" or box beam, you will. Get instruction on how to best locate and screw into a rafter.
About getting the wiring from the roof solar panel down to the inverter where you'll need to make the final connection. Assuming your fridge is not in a slideout, it has a roof-top propane exhaust vent. Run the solar panel wires down that chimney. The wires come out the bottom just inches from the inverter. The exhaust vent is not going to damage the wires as there is little heat. The chimney is for propane fumes.
This was told to me by the PC dealer I bought my rig from in 2007 when I was considering large roof top solar panels. I did eventually decide against the investment.