Closing up the RV for 3 weeks,,,thoughts!

I assume that flying is not an option. And I'm not sure why it will take three weeks? Maybe three days depending on where in TX you are parked.
Close the slides, put everything away you can think of outside, Be sure the propane tanks are full, and turned on, leave the refer on unless it is empty, turn off the water, drain the holding tanks, Place an electric heater inside on low,lock the door and go.