r4man2,
The solution EVDO recommended to you is a good one. I passed on that particular router as I needed the VPN capability in order to access my office. Most people have no need for that feature. The Kyocera fits the bill for most and it's pretty inexpensive.
As for power draw, I can share my experiences with you. I'll give you the short story answer first. Keep the router/cell booster etc. on a switched DC circuit. That way, they won't be drawing any power with the ignition key off as the circuit they are using will not be on. No power draw = no ill effect on batteries.
In less than one year, running a mobile router with AirCard 24/7, I killed my two OEM starting batteries in my RAM. Thereafter, I added a 3rd battery (AGM sealed type) and a dual battery isolator to isolate the 3rd battery from the dual charging batteries and still charge them all from the alternator. Much better.
Another thing I started to do was to move the router into the trailer once we setup camp. Mine came with a 12 volt DC adapter and a 110 volt AC adapter.
Jim