We have the 760 LMT. I update mine every few months, it's easy. Google maps will not help with RV situations. We have 38' motorhome plus tow a Jeep. All that being said, yes you most definitely should check routes ahead of time, I do use Google. The Garmin 760 has been okay. None of them are perfect. I've had GPS built into my truck for 10 years. They are not problem solvers. They all have filters, but simple ones. If you are not a little familiar with the area ahead of time you are guaranteed to go "out of the way" on some routes your GPS says are the best. As far as functionality and ease of use I found the Garmin to be okay.
2015 Georgetown Forest River 351DS MH
2008 Keystone Cougar 293SAB 5th Wheel
2007 Jay Flight 30.5RLS 5th Wheel
2006 Jay Flight 29BHS TT