Hi Dick,
Been through what you're going through. I actually decided agains a dedicated GPS because I couldn't find one that had the screen size and maps etc I was after.
I decided on an iPad (they have them in all sizes up to 12 inches, just make sure you get one with the GPS chip) and the RV version of CoPilot. It does all I want it to do and has "off line" maps as well, not using any of my data plan. It also offers just on time traffic information that only takes up a smidgen of the data plan.
I know you're looking for a self contained GPS and don't mean to get off subject. Just though I'd mention what worked for me!
Harry