I LOVE this idea.
I have been contacting the renters directly to ask about their travel plans. Where are they going, what will they be doing, etc. This allows me to better understand their trip and possibly provide some insight, based on my own experiences, or provide maps and a list of suggestions for the places they are going. It also gives me a chance to spell out some of my rules, which may cause me to lose the rental in some cases but is worth it versus receiving a dirty banged up RV at the end of the trip.
Just a thought, some travelers prefer a more spontaneous trip and only know a general sense of where they are going and may change their mind in the middle of the trip. How would this addition to the map account for this type of traveller?