Luckily I have a friend who doesn't mind checking water and dropping in a cricket or two. Almost all of my Ts have a water dish - even if just a bottle cap. Here in TX, the humidity can vary from 100% one day to 25% the next. Since I must use a space heater at times, I manually mist the air if I am in the room, but I also have several dishes of water sitting out near the T cages (those that like a wee bit higher humidity). The evaporate slowly, and I have actually noticed a 5% or so rise from when they are not there.
Best thing is to have someone go over 3 times or so, and tip some water into dishes for you. (I'd hope you have someone collecting the mail, turning on and off lights, etc, just to keep an eye on the place.)
For each enclosure, soak several paper towels (seperately for each towel) and wad them into balls, toss them in. Overfill the water container. This is how they ship the Goliaths, who require high humidity - I'm assuming it may help.
Thanks guys! And Fran, it's mostly my little .5 in. Tapinauchenius subcaeruleus I'm worried about the most, and my h. spinifer. By the time I leave, I should have the tapi, an LP the same size and a b. jacksoni, also the same size.
I put the tiny slings in a tupperware container which has water in it, and a lid sitting 3/4th on. This keeps the humidity up but allows for air movement. I also lower the temperature to the upper 60's. And of course the water.
I've been able to be away for four weeks and never lost an animal. Of course it does depend on your species and their sizes.