There are several options, ONE may actually work...
First: Atwood makes a "10 gallon water heater that fits in a 6 gallon hat" by heating the water much hotter than normal then mixing cold and hot via a thermostatically controlled valve it can deliver 10 gallons at 120 Degrees..
Downside, uses a lot more energy, and is much more complex. but that is 2/3 more hot water.
Second. Some water heaters you can use GAS and ELECTRIC AT THE SAME TIME and if you use the cut off valve when soaping up this can last much longer.
Finally you can get a "TANKLESS".. now these play to mixed reviews and there is no practical Electric option for tankless (You simply need too man BTU's. on 30 amps you do not have enough watts.. So they must burn propane)
Theory is unlimited hot water
Final option.. NOT RECOMMENDED FOR INDOOR USE unless you have very good ventelation.. Coleman portable water heater with optional shower attachment
I actually had one of these, kind of wish I still did... Worked great.
It too burns propane and is in fact a "Tankless" water heater.