1. If you stated your post correctly. YOU NEVER LEAVE THE DUMP VALVES OPEN WHEN CAMPING. ESPECIALLY THE BLACK TANK. COLD or Hot weather
2. If you doubt this, look up POOP PYRAMID.
3. Tanks, the down pipes are supposed to be sealed on top the tanks. As another posted, fill up the Black tank full and then slowly fill the rest of the tank THRU the toilet, until water comes up into the bowl. Once you do this, if the vent pipe or the toilet pipe and gasket are NOT sealed you will see water drip on the ground. No leakage, your system is OK.
4. IF you camp with the Black Valve OPEN, you will have a poop pyramid. THESE SMELL HORRIBLE.
5. Your relative did nothing to cause this problem.
6. If you have a Black Tank flush, this may be a leak area, but the fill test will confirm this. Doug
PS, tanks have NO overflow system