We use Microsoft Streets & Trips 2013 which last time I looked you could download to your laptop from Amazon for $20 - then you can order a GPS receiver - or buy the one from Amazon the MS puts out. This comes with all the roads in North America including many small towns in MX. There is also a GPS from Garmin that has a 7" screen we are looking at that can be found for about $216 that is already loaded with MX maps - our experience with Garmin MX maps are they are quite detailed - but we still prefer S&T for planning.
Several years ago I created a basic tutorial for S&T that can be found at . . . http://www.rvtechlibrary.com/electronics/gps_streets_trips.htm . . . the last time I looked. It is somewhat outdated now but can give some idea of what can be done using S&T. I have updated the tutorial but it is not posted. I have lots of other info if you want to PM me. See our blog too. Some like to plan and some like to wing it. I find I enjoy planning when not traveling as it takes me where I want to be. I also love fooling around with maps. Technology has made computer maps much better than any of the old printed maps.
I have heard these are the best GPS receivers for a laptop - mine is older - the SiRF Star IV is the latest technology . . . http://www.globalsat.com.tw/products-list_new.php?menu=2&gs_en_product_id=2 . . . these can be found on Amazon in the $30 range.