When we lived in our toyhauler in Colorado during the winter I discovered that the temperature of water coming into the water heater had a huge effect on how long the hot water would last for showers.
I installed an on demand propane water heater inline on the input side of the water heater. This used 2 D batteries for ignition and connected to a regular ground mounted bbq style propane tank. It would take our 40 degree water and raise the temp into the 70's. This allowed us to take pretty normal showers.
You could also look for a small electric unit like they use under the counter or cabinets in many commercial applications and put it in the basement or even under the trailer you would just have to figure out how to get electricity to it.
Ty
2018 Ram Cummins Mega
2007 Ragen Blackhawk 36-05