First of all, Everglades Rats are snakes from very humid areas of Southern Florida, so they don't come from a dry climate. However, they are fairly tolerant animals and easy to keep so I suspect you can keep them easily at humidities higher than 50%.
I used to keep everglades rats when I lived in El Paso, Texas and the humidity was frequently under 20% for much of the year. The snakes did fine as long as they had access to drinking water at all times.
Trans-pecos Rats on the other hand require a low relative humidity. They are native to the El Paso area, but are nocturnal and live in cracks and crevices during the day where humidity is slightly higher. They generally do great at a humidity of 50% or less, although I have a pair now that is doing fine even though I now live in a fairly humid climate.
Any colubrid care sheet can give you info on keeping Everglades Rats, they are easy.
The best care sheet for Trans-Pecos Rats is probably the one at VPI -
VPI Trans-pecos Ratsnake Care Sheet.
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Chris Harrison
...he was beginning to realize he was the creature of a god that appreciated the discomfort of his worshippers - W. Somerset Maugham