Gaia GPS is a pretty good app for what you are talking about. It doesn't give you turn by turn voice directions or anything, but it does give you access to a number of open source maps (including topographical, which is awesome in the mountains). You can use it online, or select an area of the map, and download it to your iPad so that you can access it when you are out of 3G service. (These maps are huge, which is probably why I couldn't find any apps that just downloaded everything onto your iPad.) You can manage the maps you have downloaded - delete ones you don't need anymore, etc.
We are looking for property in the NC mountains, and kept losing 3G coverage, so I searched out this app to help us. It might not have every truck stop marked or anything, but it does have a ton (10,000,000) of waypoints that describe points of interest, and you can drop pins to mark routes or make notes. It also has a cool function if you are hiking. It tracks your trip speed, the amount of time you were walking vs resting, the total distance climbed, etc. You can also see your altitude at any time. It's very cool, and I haven't even had time to figure out how to do everything on it.
I'm not sure how up to date the maps are (it uses My Topo, I believe), but I do know that my development doesn't show up yet. So there may be some issues in some places. But that's true of almost every map I've ever used!
April
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