Sometimes we do keep odd things even indoors. And if that is what the vendor suggested ... they often know best. Last year in September I bought a Hardy Musa Basjoo banana and they said keep it inside.
OK ...so I kept it in my basement grow area and it did okay. Planted it out on Mothers Day and it grew to 7 feet. This winter I am over wintering a Hardy Windmill Palm indoors and will plant it out next Spring. It is in my unused back bedroom.
I see no need to place it in the bathroom. Frankly - the warmer will NOT necessarily be the better. That plant is hardy up to Zone 6 I believe (in general - I have no idea what you have). Just do NOT give it any fertilizer or any other stuff over winter. Just some water.
It is better not to put stuff in a garage in pots directly on the cement - that transfer cold big time.
I am sure it will do okay. Just remember - crape myrtle are late to come out of dormancy. And when you bring it out after last frost, don't place it directly in sun. Do it gradually. After you get it planted and it has been in the ground for a month or so, I would give it a half strength shot of fertilizer. Or better yet - when I plant anything in the ground, I give them some Quick Start, UpStart, Plant Starter or similar. Helps prevent transplant shock.
CM like a lot of sun and check your care information for siting - they can get pretty big in Zone 7. A couplke of years and it will be awesome.
