The simplest and easiest way to get service on the road is with a 3G setup. The major phone carriers in the US all offer pretty descent data service. Verizon probably has the single largest, although Cingular is getting close. T-mobile is almost non-existent. Check their coverage maps and compare it to where you want to go. Also, note that you don't need to be in the 3G/high speed area to access the internet. The equipment will work anywhere you can get a cell phone signal. If you are out of the 3G area, it falls back on the standard 2G/Voice system. Expect the speed of this service to be slightly better than an analog phone modem. In other words, if you want to email and surf pages that are mostly text, it's perfectly fine. If you want to do a lot of video, then the 2G service will be very frustrating and take a long long long time to upload or download the content.
The best thing you can do for coverage, if you're planning on going on the fringe is use an external amplifier/booster and put a reasonably high gain antenna mounted high on the RV. With this, you'll pull in a good solid signal where the standard issue equipment won't get anything. You can find them any number of places online. Make sure it works with the high speed/3G service - usually on the 1.9 ghz band. You can get even better distance if you put the antenna on some kind of expendable telescopic pole. An antenna mounted high with a descent amplifier will get you many miles of added coverage.
If you are really really out in the boonies, like in the badlands of Death Valley or the isolated ravines of the Rocky Mountains in Wyoming, then there is no service that you will be able to rely on other than satellite. There are two basic kinds of satellite internet: Mobile satellite services and DBS type satellite systems. The mobile satellite services include Inmarsat, Irridium and Globalstar. These systems are explicitly designed to operate in motion and to be used in extremely remote locations. Irridium, for example, works on all points on the earth, even Antartica. The equipment is small and portable. There are a couple disadvantages: they are generally slow, and in the case of globalstar and iridium, they are absolutely snails pace - think 2400 baud under good conditions. Inmarsat has a service called BGAN, which is about 128kbps per channel. It is also astronomically expensive.. just stunningly expensive. Think 5-7 dollars a megabyte expensive. It's so expensive that if you can rack up over a thousand dollars in traffic just by casual internet surfing for a couple of weeks.
Then there is the other kind of satellite internet: the DBS/VSAT type. This type is really designed primarily for residential and small buisiness use. It is offered in areas that are lacking DSL, FIOS or Cable. You wouldn't ever want to install this in an area that did, because its generally inferior to those kinds of connections due to latency. The latency is not so bad that you can't surf the web - usually it's acceptable but don't expect to do online gaming with it, because it has ping times of 400+ ms. The equipment for this is small dish and a residential modem/gateway. It's on par with DirecTV or Dish Network in terms of the size of the dish. It costs anywhere from 50-150 dollars a month for the service. Although the dishes are generally intended to be fixed mounted, you can attach them to a tripod or something for portability. If you want to use it while you're in motion, then you can do that too, but expect to pay more than $1500 for an in-motion tracking system.
If you go with satellite, I'd recommended Wildblue, but you can also look at HughesNet and Starband. Expect to pay a few hundred for equipment and maybe 75-100 a month for the service. It will work anywhere in North America that has a clear view of the South.
Wifi? You have got to be kidding me. If you mean to use it within the RV to allow you to move freely with your laptop, then that's one thing, but to actually connect to the internet? You may get a signal if you're in a city. You're unlikely to get any signals at all on the highway. You may get a brief as you speed by a Starbucks or a home with open network access. Expect it to last a maximum of 30 seconds. Seriously, wifi only works for a few hundred yards with most equipment. If it's a high power system with good antennas and a clear line of sight, maybe a mile. You will get absolutely no wifi coverage at all in anywhere even slightly remote. If you're in the highway in Montana or something, wifi will be avaliable just about zero percent of the time.
I would seriously recomend 3G. Add satellite to that if it is absolutely critical that you get online even in the most extreme of isolated locations.