I've used a device from a company named Ultradata that I have found extremely useful. Its basicly like a calculator other than it contains a lot of travel information relating to Interstate Highways and U.S. highways. Escentially you enter in your state, driving speed, Highway or Interstate, direction of travel, and then a catagory.
The unit will visually feed back the next gas station, resturant, repair facilities, hospital, camp ground, highway intersections, cites and more. Once you put in your initial information you just have to input your mile marker to bring the unit up to date as to where you are at. For example we use it to determine the next rest stop, gas, and whether a resturant has RV parking.
What's also interesting is that it will allow you to do a search based on names. Say for example you only want Shell gasoline. You can key in Shell and it will find the next Shell station based on your current location. It will even give you directions if you wish.
Obviously information in the unit is subject to change over time so you do have to ocassionaly replace the unit to keep the information fresh. Apparently Ultradata has released a unit that has the ability to voice back information. Unfortunately that unit is not equipped with the RV database. If any of you have used the voice unit please let me know. My current unit is over 3 years old and I'm contimplating purchasing a new one. The voice system is interesting but I do like having the RV information available.
Here is a link to the Ultradata site.