The GPS service is free (the part of the system that gives you a latitude and longditude). The US military put the satellites up and allows free use of the signals they generate (actually, we have free use of some of the signal, higher precision signals are encrypted for military use only).
The GPS device takes the free GPS signals, computes the latitude and longditude, and overlays this position on a map which is in the GPS unit itself. What is sometimes charged for is map updates. Every year or so, map data suppliers make newer versions of the maps available. This would include new roads and updates to existing roads. GPS manufacturers make this data available for users to download to their devices at a cost or sometimes for free. Rand McNally makes these updates available for free if you have the lifetime map update version of their RV GPS devices.
How often you need to actually update maps is debatable. Annual updates are probably more than enough for most folks and some folks never bother to update maps. Rand McNally has made other updates available for free with the lifetime map option. For example, when I bought my unit, the Woodalls campground info wasn't available. After Rand McNally did the deal with Woodalls, they made the campground info available in a free update about 3 months after I bought the unit. This isn't just the position of a campground as per some other units, but an idea of facilities, cost and Woodalls ratings.
GPS units can be bought for $100 or less for a basic small screen and basic maps. The Rand McNally unit sets itself apart by offering enhanced routing (takes into account low clearances, narrow roads, bridge weights, etc.), RV related points of interest such as fuel stops, campgrounds, pet hospitals, pet friendly sites, etc. and a larger than average screen. Its cost is more than a basic unit, but there are so many more RV related features - for me it is worth the extra money.
There is one other RV oriented unit available from Magellan - I can't recall its name right now, but it looks pretty good too. Before purchasing, I compared the feature set for both and concluded that the Rand McNally unit was the best value for me - but each person has their own priorities and needs. For folks that only need basic positioning and a basic map, my unit is overkill.