I live on the Gulf Coast, a native, in a virtually identical climate - maybe wetter. People here have model RR's in the garage quite often. You must have A/C; insulate the garage door; if not, get another hobby or move to Colorado.
Otherwise, contrary to some beliefs, humidity is not fatal, nor will it come and attack you.
I personally have a separate RR building; small window unit A/C and insulation (except under the floor; the building is built on piers, about 1.5 ft high; common here, good for a breeze/summer heat dissipation). My trains - TMCC and so on - are fine, run fine, some are 20+ years old. I do not run a de-humidifier. I run the A/C only when I'm in the building.
I'll take our "natural" humidity to that found in some Northern basements any time. And - no sump pump!
