danoren, given what you say is your most important issue then you really have a pretty short list of devices to consider. Your options are: Garmin delz, Rand McNally RVND series, Magellan RoadMate RV, and a new TomTom VIA 1605M RV.
The Garmin, Rand McNally and Magellan all use the same map provider: Navteq. Theoretically there will be no difference in the map. There WILL be though in reality because each vendor will have different timing as to when they buy updates from Navteq and release them to their customers. There are also differences in update frequency. Garmin is quarterly. Magellan is twice a year and Rand McNally is once per year.
TomTom owns their own maps. The main difference with TomTom is that they tend not to have the exact wording of freeway signs. Otherwise, their maps are pretty much on par with Navteq these days.
The Garmin delz is a truck navigator. You can it for an RV to the extent that you can put in the length, width and weight, but you won't find any RV "features" beyond that. You could also get stuck with some truck speed limits that don't apply to RV's in some states.
The Magellan and the Rand McNally are designed for RV's and therefore will have a lot of extra RV related features. They may not be important to you but they are there.
TomTom is new to the RV business, though they've had a unit in Europe for quite a while. Their big advantage is their routing database and it's IQRoute engine. The route selection and the ETA are stellar for the most part. It is unknown in the US how much RV specific data their map database has. They are unproven in that area in the US.
If you want voice recognition the only option is the Garmin.