If its not splashing from around the door or over it then you have to do a few simple tests to try to isolate the problem. First pressurize your system from mains to see if its a tap, having first dried all earlier tell tales, no leaks then drag a garden hose inside and run it into the sink check for your leakage, run it onto the shower drain this will show if its a drain leak, if not then do same with your van taps, the idea is to isolate the individual possible sources before you dismantle the whole van.. If the drains are still original Jayco you could be getting overflow from the kitchen sink and/or if its a combined ensuite the Thunder can could be leaking, not the yucky stuff but flushing water...... Its not uncommon for tap fitting to come loose behind the wall, its not a hard fix most of the time. Oh and have the van leaning so it drains the way you want, quickens the discovery of possiblities.
Pics are always handy and as much detail as possible as the variations of set ups are many..................... Welcome to the mob, we are the friendliest group online around..................but do keep your thread alive or it will be passed by due to the high number of one post wonders..........