comments:
If you really want the MIL to be happy replace that POS dual-knob faucet with something like this I don't use the term 'greatest thing since sliced bread' often but it applies here: the dial on the left controls water volume and the one on the right controls water temperature. There's a push-button if you want to override the Hot above 100• - really a great improvement over the stock 1900s style control.
Your biggest issue is likely the water heater bypass valve being in the wrong position for midway between positions. It was left in this position by the RV dealer when they winterized the system. Read your manual. Get a better shower head. Replace the shower valves with one of these https://www.amazon.com/dp/B098NGP43C?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_1 Do you have two water heater switches. One marked AC or Electric and one marked Propane or Gas? If so, you can use both at the same time while showering. When it comes time to shower, I turn on the Propane/GAS Water heater switch and can get a 10 minute - 12 minutes hot shower.