What tent are you and your wife looking at?
I'd suggest staying with a hard shell. Faster to set up/take down, sturdier in high winds, minimally better MPG. Downside is they are usually a little heavier.
Never had a problem with GPS. When we're off roading we use the infotainment nav screen map with "bread crumbs" feature to track where we've been in case we need to turn back and find our way out. Never had a problem with tracking us, regardless of which direction we're pointed.
Doesn't affect...
It depends upon how far forward you mount your RTT.
I have the large iKamper on a Gamiviti rack. The RTT is mounted so the front is aligned with the front angle of the rack. The iKamper is FULL length and completely covers the shark fin antenna.
With that being said AND how covered the shark...