The fast way is to perform (or have somebody perform) a Pressure Test. The better RV shops have an (albeit expensive) machine (variant on furnace blower) that creates a little positive pressure inside a closed RV. Then you (or tech) goes over the outside with bubble solution in a spray bottle. Just like testing a tire for a nail, but much lower pressure. Nice thing is, the bubbles show the SOURCE of the water, not where it may appear inside.
That said, a good RV tech knows where and how to look, and generally will find the source.
Naturally you can go on your own hunt and if the source is readily visible, clean the area, remove loose sealant and apply non-leveling Dicor Sealant. Please DO NOT USE SILICONE products. They don't hold well and leave a residue that nothing will stick to after.
Inspection and re-sealing are annual requirements under warranty for new coaches. We all need to do it. RV's are subject to heating, cooling, flexing, sunlight and freezing. All of those compromise the seams.