Tips for storing 5th wheel in Phoenix area in summer

I have 10 Park Models, one 40 ft 5th w, 3 TT several houses that most sit in the sun here in Yuma during the summer heat. Several are under metal awnings.I put about 7 or 8 one gallon milk containers thru out each one of the above. I seal up the windows, cover the drains in all sinks, bath tubs etc. Cover top of toilets also top of toilet seats etc etc. I have been doing this way for all my rentals for 13 years. Very few problews other than dust and dirt get inside some. I also drain all the water heaters