The LNB is located at the focal point of the dish so that is the correct location to locate the antenna. I would doubt that a roof-mount intended for a satellite would aim low enough for the wifi signal without modification.
I ran across this site today and it looks like an interesting project for those interested in extending their WiFi range. This DIY project involves re-purposing a satellite dish as a WiFi antenna.
I haven't tried building one but it looks like an interesting and not too difficult project.
I have a Yaesu FT-7900 (2m/440) that I take in the truck and an FT-840 HF rig that's small enough to take along for use in the trailer or on a picnic table. I haven't taken the HF rig camping yet but plan on doing so on our next trip. I have a screwdriver antenna that I plan on mounting to the ladder on my TT along with a 10m dipole to throw up into the trees.
I'm looking forward to getting away from some of the power line noise and other interference that we have here in suburbia.
I'm working on my wife to get her technician license so I can use a handheld to communicate with her when I'm away from camp.