As stated above. Also, many times, the water is entering at a location not directly above the water you found on the inside. If you can find a way to pressurerize the interior of the rig (with air), soapy solution squirted all over roof seems will show bubbles...and you will have found the leak. Usually, this is along an edge. As Dave above said...that too is a highly suspect area.
I have heard of guys doing the air pressure technique with a large shop vac. as follows: Shut all windows and doors/vents. Open one window sligthly and make an attachement point/seal from the outside (cardboard-duct tape array), attach vac, set to exaust mode...go up and check seems with soapy water. I have not done this but some RV places do it all the time. Some others just goop on some sealer in the suspected area and charge you...until it leaks again sooner or later.
If you find the location of a suspected leak, DO NOT just dab silicone or "any ol' stuff" in there. It will not work and will make the proper repairs later on, more difficult. You can find the proper repair for your type of roof on these forums and sometimes do it yourself. Even if you want to have it done by a shop, read up on it here and ask around so that you will possibly not fall victim to "Ahh, yeah...you need a whole new roof...that will be 2500 dollars minimum."
You probably do NOT need a new roof if you have caught the problem this early. If you are handy or know someone who is, AND will learn what the proper repair is, you can buy the materials and fix it most times for pennies on the dollar.
Start reading up on it. Good luck!
1984 Midas Freeport 24' Class C
Ford Chasis, 351W C6 @ 118K miles