Cold Water...BRRR

I am still waiting for hot water in the guest house.  We have plenty of cold water.  I guess I should be grateful to have a toilette that I don't have to empty every 2 days.  The shower is there and it sure does look pretty, but it has no hot water, so it's completely useless.  We still have to use the RV for showering.  It takes the hot water in the Rv about 15 minutes to warm up nicely.  The tank holds 2 gallons and will give about a 7 minute shower.  We have to be quick when washing hair.
My plumber has the inspector coming tomorrow afternoon to check the gas and water heater installation.  If all goes well he will sign off on and green tag the gas line, which means that the gas company will be able to turn on the gas and get my tankless water heater running.  It;s a beast of a machine.  It is large enough for 2 bathrooms.  Since the guest house house only has 1 bath we should have plenty of hot water.  I am fiening for a long hot shower in a tub that I can lift my foot in without banging my knee on the wall.