If I'm leaving my trailer for awhile I pour some rv Antifreeze into the toilet bowl to keep the seal I haven't checked but may evaporate slower. Water in the black tank may evaporate out through the roof vent, but it would be at a pretty slow rate, just a a little more water.
TV: 2013 F350, Lariat, 4x4, CC, LB, 6.7L Powerstroke
FW: 2013 Jayco Eagle Premier 361 REQS
Additions: Onan 5500W LP Generator, Surge Guard TS,
.......Roof Satellite Dish, Vent Covers, Bedroom Vent Fan,
.......Rear Reciever, 12V DC Outlets (6), 320W Solar & Controller,
.......4 Lifeline AGM Batteries (600 Ah @ 12V),
.......Stereo Upgraded, Interior Speaker Upgrade, Amp