RV Tub Will Not Drain Properly

I have a 1977 Layton travel trailer and it has a bathtub/shower combo, and the tub suddenly started filling up with water, not draining and I cannot locate the problem. I checked the outside water lines and there is water all over the ground and a small continuous drip. It has never happen before, I am puzzled and have very little knowledge of RV repairs, please give me an idea of how I might be able to fix this issue. Leslie R., (Yakima, WA)

Leslie, I almost hate to ask this, but...have you emptied the gray water holding tank lately? The tub will likely drain into a separate gray water tank, but in some cases may drain into the black (solid) waste tank. I'm not sure how it is with your Layton (I can barely remember what I did yesterday, let alone something Layton did back in 77!) 

If the tank is empty then the problem could be a plugged P-trap or blockage anywhere between the tub and tank. A venting issue for that holding tank will also create drainage problems. If the vent stack has slipped deeper into the tank (below the level of the contents), the tub will likely back up or drain very slowly. But the fact you have water leaking tells me you probably have a full holding tank and/or a failed termination valve (dump valve), and it simply backed up into the tub.  

If, by chance, the shower drains into the black holding tank, shared with the toilet, and you only evacuated the gray tank, check to make sure both holding tanks are completely empty and see if the problem persists. Also, with both holding tanks empty you can easily find out which tank the shower drains into by leaving the termination valve open on one tank, then run some water through the tub and see which tank it drains through. It is permissible, by the way, to have another fixture drain into the black tank and many floor plans mandate this unfortunate feature. 

It's also recommended to upgrade your termination valves to electric gate valves. Check out the Drainmaster electric valves.