I did some basic engineering calculations on these things and I'm very skeptical. Someone identified a 6.9 gpm flow rate which works out (6.9 * 60 * 8) to about 3400 pounds of water per hour. Assuming an inlet temperature of 45 F and a temperature at the shower head of 115F, a heat transfer rate of 235,000 BTU/hr would be needed for continuous operation with no degradation in temperature (3400 * 70).
This works out to about 92 horsepower (235,000/2545) which converts to 68 KW (92*0.746), or 60Amps at 115V.
All the S&B versions I've looked at have needed 220V wiring - maybe this is why?
I don't have data on the calorific value of propane so I don't know how much you'd have to burn per minute to get the equivalent of 68 KW.
Any other engineers on the forum? Get your slings and arrows out - I may have dropped a 12 or a 60 in there someplace.
If DW wants the hot water to last longer when she showers, maybe a much bigger (40-gallon residential type) water heater would be a better idea.
Frank and Eileen
No longer RVers or FR owners