As mentioned before, the temperature of the incoming water is a huge factor in how long it takes any type of water heater to recover. The incoming water temperature obviously is dependent on season of year and location. The data sheet on the Suburban 12 gallon model states a recovery rate at 10.1 gal/hour on gas and 6.1 gal/hour on electric. In theory that should make the recovery rate 16.2 gal/hour running both. I don't know of any long term damage from running both fuel sources, if it were dangerous, they would have a lock out so you couldn't run both at the same time.
What they don't state is what incoming water temp that's based on, and what the thermostat is actually set at. As you know, the thermostats are not adjustable unlike the one in your home. I'm working under the assumption the recovery rates are based on 55 degree incoming water which is kind of a national average, and the thermostat set at 130 degrees. That is plumbing code setting in many areas of the country. Under these conditions, it theoretically can reheat the entire 12 gallons in <45 minutes, which seems a little lengthy to me.
Most people will instinctively adjust shower water to about 100 degrees, which feels comfortable. If you have a shower head that will pass 2.5 gal/min and allowing for the amount of cold water mixed in to get down to 100 degrees; you should get a 6-8 minute shower before the water starts cooling very much. The burner and electric element will turn on long before you draw all of the hot water out and the shower is still running, but I doubt it will give much heating at that flow rate.
I briefly looked at a tankless model for our RV; we have a tankless in our home and like it very much. My conclusion was there were too many down sides to a tankless in the RV. The first being cost; a good tankless is >$1K plus the install cost if you don't want to DIY. Another is the fuel; you have a finite amount of LP onboard and you need it for other appliances like cooking and furnace; with a tankless you have no option other than LP. Unless you're camping somewhere that has metered power, the cost of the electricity is built into your rental cost. The tankless models have a large burner; the better ones will throttle the burner down to match the flow rate, but it's still big. There is some maintenance required on a tankless like periodic cleaning; if scale is allowed to build up in the heat exchanger it will greatly reduce the efficiency. If you're bouncing around the country, you have no idea of the incoming water quality. You can filter out a lot of sediment and most of the chlorine, but only a water softener will reduce the calcium content. Another consideration is freeze protection; if you're camping during freezing temps any time you are not running hot water the tankless is pretty exposed to the weather and still contains a small amount of water in the piping and heat exchanger that is subject to freezing. Some models have a small electric element to prevent freezing.
I also use the "turbo" method on my water heater. I fire it up on both sources when I initially park the rig so I have some good hot water for hand washing after I hook up all of the utilities. I normally just leave it on electric only until shower time, and turn the gas back off after we've showered. My recommendation would be give the tank model a good trial before you opt for a tankless.