Actually, as long as you meet one criteria, you can take as long a hot shower as you want. You need full hookups. Hook up the sewer and leave the gray tank valve open so the water won't fill up the gray tank. (Do not leave the black tank valve open.) Hook up to shore water so you don't run out of water in the fresh water tank. Turn on both the electric and propane heating elements on the water heater.
You will have unlimited water, unlimited drainage, and since the two heating elements will keep up with the water usage, unlimited hot water.
But that does prevent boondocking.