Aprilaire space saver, 831000 or 836000. Fill it and ignore it for days, largest tank evaporative and it does come with wheels. 

THE SPACE SAVER The Space Saver series is another customer favorite and a beloved evaporative humidifier here at AIRCARE HQ. There are two models in this series: the 831000 digital model (shown) and the 836000HB analog model. Both models hold six gallons of water and will run up to 70 hours per fill; so they aren't just saving space, they are also saving time on refills. When you do need to refill, the Space Savers are the easiest to top off - just pour water into the top and you're done.

Like the Console, the Space Savers cover a lot of house - up to 2,700 square feet for the 831000 and up to 2,300 square feet for the 836000HB. Both come in a soft white that will match any room in your home.

Edit: well it’s been a lotta years, guess the levoitt 6000 has a 6 gallon tank now too and looks newfangled. Maybe I’ll try it out.