Could someone please explain what the following files are,and what they do?
FPA
FTR
HNR

I have them in my contents but dont know what they do and my sat nav operates well without them.

Also could you tell me what is needed to run Primo Truck on my device so i can input dimensions of my RV and calculate suitable roads.Thank you.

See here:
iGO-Primo-Enhancements.
For Truck, you need the FTR files that match your map files release, and put them in the same folder with maps...{Primo}\content\map.
Then go into settings for the vehicle type and make the correct entries for size, weight, etc.

Thanks,that is a great help.If you input a vehicles dimensions and you don't have for files in your map folder,does the software just route as if you are driving a car?

Yes. Sort of. If the FTR files are not present (or do not match the map files), anything you enter in the settings for "Truck" are just ignored.
With working FTR files, you can switch back and forth between Truck and regular Vehicle for routing, etc.

Ok understood.Thanks again for all your help.I really like primo a lot better than IGO which I have used in my RV for years.Still trying to get to grips with it all.Next step is to try again to get the map changer to work.