I had the RM 7710 and really liked it for the 3 years it worked. Then it died and RM wouldn't touch it. I also had an older Magellan (not the RV specific unit) until it died this year. I purchased the Garmin 760 in Oct and it has worked fine so far for both the car and the RV.
Differences - the RM talks a lot more than the Garmin (the Garmin dings instead of telling you some things. One of our friends found the RM annoying). Each has different POI's but both have gotten me to campgrounds. The RM had military campgrounds, which is useful to us, but I was able to download a POI file for the Garmin. Garmin is easier to read, not as much glare as the RM. They are both 7 inch screens. I hope Garmin has better customer service but I won't know that for a few years, hopefully.
The Magellan I had is about 9 years old. Because I didn't have lifetime maps on it, it soon became obsolete but it worked as a spare for a number of years.
Basically they are all similar, it just depends on which you like. It took me a little bit to adapt to the Garmin after having the RM for 3 years. Now, I can't tell the difference.