Since I don't know about the 19' plumbing, I can only say to check the fittings under the sink for a leak—run water into the sink and see if you can find a leak. Fill the sink part way and then let it all drain at once. The P-trap under the sink or one of the pipes attached to it may have a loose fitting that needs to be tightened. It can be done by hand. Ours started to leak after a few years.
A drain system is not designed for high pressure, so a loose connection that gets a sink full of water at once can leak if not tight enough. A full tank could back up and put pressure on a fitting, but it usually shows up in the shower first since it is the lowest point in the system. I assume a 19' has a shower. If you have one tank (grey and black together) you don't want to see the results of a backed up drain system.
I don't know if this helps, but loose fittings are not uncommon in sink drains—either not tightened sufficiently at the factory or loosened by travel. It happens at home too because sink drains like to loosen over time too. A loose fitting is the simplest thing to check.