The simple way (you may not think it simple) would be to go to Texas first, do everything needed to establish domicile, before purchasing the motorhome. If you want to buy the motorhome in another state, you will have to take it to Texas to title it (may be VIN inspection for that if it is used) and register for tags (safety inspection for that, before first registration and annually).
Some of the process meshes together. You might need a Texas driver's license and address to get a Texas title, and probably need the home address to get a driver's license. Neither of these requires domicile (a Texas address only), but does help establish domicile. Liability insurance will be governed by the laws of the state where the motorhome is licensed and by where it is "garaged" if that is different.
For Texas, it might help if you talk to Escapees, they are in the Texas domicile business.
I don't know that Florida is a particularly better RV buyer's market than Texas, but maybe you already have your eye on a particular motorhome in Florida.