Comment CradlePoint MB1000/Kyocera KR2 + EVDO card (Score 1) 438
...works great for me. I have my EV-DO Kyocera KPC680 (Verizon) card with me everywhere and a Kyocera KR2 camps out in the car with its requisite DC-DC power adapter. Install a nice high-gain antenna (Wilson [eBay] has some decent +9/+13dB gain antennas for EV-DO/1xRTT and GSM/W-CDMA/UMTS bands) on the roof of the RV and make sure you have a decent groundplane if your roof is fiberglass (no worries if it is aluminum) and you should be fine for something like 95% of US roads traveled (you will drop to 1xRTT/EDGE in some areas, but 3G (EV-DO/W-CDMA/UMTS) is pretty well covered on most interstate routes, based on Verizon and at&t's service maps).
For Canada... I'm not too sure there as I'm a Yank and have not had a need to visit our friends to the north yet. You might be able to get a roaming data plan that includes Canada from your US cellular provider though.
FWIW, I work for Kyocera International, parent company of Kyocera Wireless.