For the "DIRECTV Genie 2 DVR Genie Server Receiver (HS17)" which is the newest Genie. NOTE: This receiver is triangular in shape and hence very distinct.
My understanding is that no other receiver can be on the same account, period. I know for a fact that this restriction does not exist for my HR44 Genie and probably not for the HR54 Genie.
The Genie 2 manual clearly states it will display a tilt error if not upright.
I speculated that the upright only restriction implied better ventilation is required and installation in a cabinet might cause over temperature.
No reason that I'm aware of that you couldn't take the Genie 2 and 1 or more clients in a RV provided you have a SWM dish.