For Apple users:
inRoute Route PlannerGarmin: There is a method of transferring the maps loaded into the GPS unit onto your computer. That will give you the required off-line maps that you seek with the needed level of detail. However, I do not have the "how-to" process at my fingertips at the moment.
BaseCamp: A steep learning curve and a seemingly horrid program.
Their help forum is
HEREPoints of Interest:
POI Factoryhttps://buy.garmin.com/en-US/US/p/135455 The Garmin RV 760LMT contains some POI's (20,000) in the US and Canada.
Lastly, this topic may have been better suited under a different topic forum to obtain wider help coverage.