I think the iPad is one of the best devices because it can replace so much space and weight. Books, map, guide books, computers, laptops, stand alone GPS units all could be left out of the RV depending on what you do of course. Make sure you get a 3G/4G model for a real GPS unit. You don't need a cell contract on it no matter if a sales person tried to sell one. The little less expensive but most common wifi iPad or really any tablet models don't have GPS. I have heard of stand alone GPS hardware to plug into a iPad without GPS but when you take in all the prices, just get the xG model to begin with.
Google just released a maps app and is supposed to do iPad version any minute too so the Apple map issue should fade over time.
Maps actually do load into memory on the iPad until they are overwritten by something else down the line. Kind of like old computer RAM memory. So if pick your zoom level and go ahead on your route, the maps stay without internet. I have used an 3G iPad for several days that way without service.