The rule of thumb given in the Moeller's book RV Electrical Systems is that you should have a 50 watt solar panel for each 100 amp hour battery. Your dash top panel is probably barely a trickle charge and would take more than a week of full sun and no load to charge a deep cycle battery. If you are thinking about using it simply as a battery maintainer you need to be sure that it has a built in regulator so that it will not over charge the battery. Would it make sence to disconnect the battery from the RV anyway to be sure that no phantom loads are present? For more info check out this website by the authors of RV'ers Guild to Solar Battery Charging:
http://www.rvsolarelectric.com/
I've requested their free info kit for determining panel requirements.
Good luck,
steve