What your probably missing, assuming your bi-pass valves are set correctly, is the need to conserve water during showering in an RV, A 7 minute shower is going to drain the HWH so it's no surprise you're struck with a cold shower.
You, AND your wife need to learn how to take a "GI shower". Get wet, turn water off, soap up, wash hair, whatever, turn water back on to rinse off. Plenty of water for 2 or more showers when you understand this technique.
Even a propane HWH will not provide hot water for both of you if the 1st person takes a 7 minute shower.
SORRY, that's just the facts of owning/living in a RV!!!