I don't think he's talking about the water heater not heating up. He's talking about the time it takes for the hot water to get to the shower head.
The problem is the hot water pipes are not insulated and just thin plastic. So water is not going to stay hot inside of them for very long. But I bet if you actually measured the amount of water used before the hot water gets there, it's no where near a gallon. Mine takes a while too. The shower is in the middle of the RV on the left side, while the water heater is in the back of the RV on the right side. I don't know how many feet of pipe are in there, but it probably takes 30 seconds for the hot water to reach the shower head. Make sure you turn it to full hot until it starts getting warm and then turn it to add cold water back in, or you'll be wasting more water.
And if the water waste does concern you, then run the cold water into a bucket until it heats up and then hand off the bucket to your wife while you take a shower. You can use the water in the bucket for dishes.