Several years ago I stopped using my iPhone & iPad for navigation. They got hot when on for an extend period of time. Plus, they burn a bunch of data when continuously used for navigation.
I've switched to using the Rand McNally GPS (rv version), and have used it all over the country, for several years, and it's ok. You just have to learn it's shortcomings. Meaning, after it got me lost in Kansas, I always look at, and have a paper map handy. Also, I always have my iPad handy so I can look at the sat view on Google Maps of where I'm going (that saved me from ending up on a dirt farm road in North Carolina).
When I had a 32' motorhome I was more casual about where I was going, as turning the rig around, was no big deal. Now at 45', I pay a whole lot more attention, as running a big rig through downtown New Orleans or San Francisco isn't fun! Both of those cases were not the fault of navigation, but because of detours .... which navi units won't help you with, but iPad/iPhone navigation will, because they will tell you current conditions.
Also, if you get stopped on the interstate, with a iPad/iPhone you can go on the state transportation web site, and in conjunction with the highway patrol web site, you can find out what's going on ahead of you and the "dedicated" navigation units don't do that.
So when it's all said and done .... use what works for you, as everything has it's shortcomings comings. At one time or another you will get screwed up following ANY navigation system ... guaranteed!
Beyond a navigation app, one of the must have apps is TUCKERS TOOLS. It's fuel station info can't be beaten. After giving you pricing, it gives you plenty of information to help you decide where to get fuel - # of pumps, how much parking, other services, etc. And, the app can help you with route planning and a bunch of other things. Gas Buddy is still a great app, but it's not as focused to big rig needs.
Plus, take a look at the new version of RV TRIP WIZARD. It's really come a long way and is now really a "must use" when planning a trip. While it won't help with navigation, on the road, it will help with navigation planning before you get in your rig to leave.