Ultimately a Trans Pecos is going to need a somewhat bigger cage than a rosy boa. TPs are about 4' as adults and they use every bit of a 40 gal tank that is
36" long x 18" wide x 17" high. They have a moderate activity level & happily use branches whereas rosy boas are somewhat smaller (females 40" & males
about 30") and don't climb very much. As far as the higher humidity where I now reside, I've been here for about 12 years with healthy TPS & rosies, etc.
A rosy would be better suited to a well-ventilated tub, if you "must", but honestly it's not fair to animals of any kind for us to own them when we cannot offer
the optimal space & conditions they need. BTW, my TP tanks are in my living room, though admittedly my house is fairly quiet (dogs but no kids).