The Gaia topo layer is the best bet for just downloading huge areas of at a time if you don't know where you are exploring ahead of time from an info vs storage aspect. That and the public land overlay are really all you need for finding campsites. I have half the Western US, Michigan of that layer and google maps on my phone and tablet downloaded.
Sat layers are a double edged sword and as time goes on I find it less and less useful. What appears on a satellite as a usable track may often in person be barely used and in terrible shape or blocked off and closed when you get there. Also some sat imagery is so old it won't have all kinds of tracks that actually exist but are not shown. This is a long way to say I used to be obsessed about trying to download as much sat imagery as I could but now don't really care that much. Sat imagery is also not going to show you info from washouts, snow level/melting etc. Those usually screw things up more in real life then what the track looked like from the sat image. I really find its a luxury I don't need too much.
There is no one source, map, or way to find dispersed sites. That just isn't really how it works.
HEMA in North America I was under the impression was more or less abandonware at this point and was not getting updated. I know the HEMA maps I used a long time ago sucked frankly but I have not used any recently.