If you have hot water at the sink for the second shower it is not a heater problem but rather the mixing valve limit on the shower is set too low to allow enough hot water to the shower head.
Newer shower valves have adjustments built in them to insure you can't scald yourself.
Remove the valve assembly and you should find a ring with teeth around the outside of the ring. Each tooth generally equals 3 degrees of temperature change.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles