(IMO) OnStar is a bit expensive - unless you feel ya gotta have it.
Even w/o it, you can still have Sirus XM and standard navigation.
(Sirius will cut you a great deal -price- if you let it lapse)
Pos solution for the OP - check out AT&T ZTE Mobley standalone $20 a month unlimited plan. (Purchase the device outright, or free w/2yr contract)
You can either plug it into the OBD port in your vehicle -and- buy/make a 110V converter for your RV/home.
BTW - in either location, *you* can set the delay for length of time it stays on - including "always on".
Threads:
On RV NetOn SKPsPlus a few more threads if you search.
Also - info on the Howardforums
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