Good stuff GatoMonte
I agree about the Guia Roja (Mexico), that was the most accurate set of maps I used on my trip. There's a bit of Guatemala included in the Guia, though the map is a bit... optimistic in spots. The winding, muddy dirt road through the mountains from Rio Dulce to El Astor is listed as a paved highway...
Maps can be tough to find, but they are out there. I began by looking in a libraria in Coban, when I asked for a map they gave me a fill-in-the-blanks classroom map of Central Ameraica!
I was able to pick up a International Travel Map of Central America in Antigua, Guatemala, that was detailed enough for the most part.
I agree with GatoMontes, get your maps ahead of time if possible.
When it comes to la policia I found that being confident but respectful worked for me. I always said "good afternoon, how are you, nice day" etc. in Spanish. Never had any trouble.
At borders you do have to brush off the tramitadores pretty quickly. Again I found playing it cool and confident worked: "No, no thanks, it's cool, I've been here before" in Spanish. There was one border, leaving
Nicaragua into CostaRica where I was told - by BMW riding locals - that the helper/tramitador was required
. Still not sure about that one.
This website helped immensely when it came to knowing how border procedures were supposed to go: Drive the Americas