Several things come to mind.
First, do you have an outside shower? If you do, make sure the actual faucets are turned off. If it is shut off at just the shower head, hot and cold will back feed into each other.
Second, are you running your water heater on propane, electric, or both?
If you have been running them on both, the recovery period is much faster, than if you are just running electric only.
Third, you might have a bad thermostat.
Fourth, (and here is what I think is REALLY going on).
In Texas you had a warmer climate than in Arizona. The water coming into the camper is warmer because of the ambient temperature outside. The water coming into the water heater is warmer, and does not take as much to heat. When hot water is replaced with cold water, it doesn't have to heat or work as hard. The water heater heats, and then hot and cold mix at the shower head. At the shower head, the cold is not all THAT cold, so mixing with the hot, doesn't take as much hot for a comfortable temperature.
When you moved to Arizona, the incoming water is much colder. Colder water takes longer to heat up in the water heater. So replacing the hot with the cold, well.. the cold is colder, and takes more to heat up, so really, takes longer, which means less hot water at one time.
Then at the shower head, the now hot water mixes with the colder - cold water. it take more hot water to bring the cold water up to a comfortable temperature, therefore you are using your hot water faster than it can re-heat. In a warmer temperature climate, the water is warmer, it doesn't take as much to heat up.
I think THAT is really what's happening.
The only suggestion I can give is ... take Navy showers. Get wet. Turn the water off. Soap up, turn the water back on, and rinse off. You can take a shower on less than 2 gallons of water this way ... even doing your hair.