Had the same thing, figured out a solution, but I STILL don't understand what was causing it!
For me, it had to do with water that had been heated in the water heater, and then allowed to cool down. Even if I drained the HW tank, the water smelled like rotten-egg the next time that I used it. so now what I do, when I get home, is drain the water heater, flip the WH bypass valve, and run some fresh, cold water through the hot water faucets, including the shower head. The shower head is the most distant water discharge in my entire water system. I catch the water in a pail, so that I don't fill the gray tank with water.
Not sure if this is the same thing that you are experiencing, but the solution works for me!
2014 RAM 2500 Laramie 4x4 Quad Cab w/Cummins Turbo Diesel, 2009 Rockwood 2607, and 3 'yaks!
300W of Suntech Solar, a Rogue MPT-3024 Controller, and a Xantrex PW2000 Inverter