The Garmin, like any brand GPS, is only as good as the source data. It won't warn you of an 11 foot bridge if the highway department does not have the bridge listed for that height.
Most 'problems' about routings and such people complain about are because of the way city, county and state highway departments keep their data.
A highway - say I-95 through Florida - isn't one stretch to the highway department. It's a series of short sections end to end because of the way they do maintenance and repair contracts.
Some cities are several years behind on reporting new roads, one city I know of in the Dallas area has reported roads as ready when they won't be built for years, if ever.
Try to look at a unit and get it in your hands. See how the controls work, is the screen size what you want. Be aware that the larger the screen, the more likely that glare will be a problem.
Try to do some searches. Try to see how easy it is to select and route to a campground 100 or more miles away. See how easy it is to find a nearby store, or one in a distance city.
An RV GPS should have the ability to set the height, length and weight of your RV so it can (hopefully) avoid routing you through underweight bridges and under height overpasses.
An RV GPS should also have a Car mode to use when in the TV or in the toad.
Garmin makes quality GPS units, and quality software. Their updated maps are good, but are huge. It takes 1-2 GB to download the new maps onto your computer and then update the GPS. They've been doing GPS units for over 20 years.
Personally, I use the Rand-McNally Good Sam RV GPS and like it. But I'm not throwing away our older 5 inch Garmin.
Once you get a GPS, pick a moderate 20-30 mile route, and drive it several times, trying the various options of the GPS. Learn how the GPS works, and what it will and won't tell you.
One last point - never ever trust a GPS to be completely accurate. You need to know which route you plan to drive and where there might be trouble stops. If you get off track, pull over and stop. Then see what the GPS is wanting you to do. See if it wants to send you down tiny streets in a big RV, etc.
Trying to reprogram a GPS while driving down an unfamiliar road in a big RV at highway speed is asking for an accident.
They are an aid, but not a replacement for maps and knowing your route.