If you can't find the model information sticker on the exterior side of the heater, how about a picture? We can often recognize them, or at least the two main brands (Atwood & Suburban). American was also a water heater supplier back in the day.
Water heaters are pretty simple, so the best course of action is to proceed with a basic function check and see what symptoms there may be. Leaks are typically in attached plumbing rather than the tank itself, but you never know til you try.
There are several articles in the RV Forum Library on water heaters. They deal mostly with newer gas/electric DSI heaters, but the LP gas mode basics are pretty much the same. Here are three of the major ones:
http://www.rvforum.net/joomla/index.php?option=com_content&view=article&id=421:water-heater-primer&catid=14:newcomers-need-to-know&Itemid=65
http://www.rvforum.net/joomla/index.php?option=com_content&view=article&id=329:water-heater-troubleshooting-and-maintenance&catid=16:rv-how-tos&Itemid=45
http://www.rvforum.net/joomla/index.php?option=com_content&view=article&id=430:elecwtrhtrrepir&catid=42:fresh-water-systems&Itemid=45