I turned mine off when we went to bed and on first thing when I got up. Had water in 40 minutes so when I took my shower.
Many RV parks when we stayed a while charged for electricity. It was usually cheaper to use propane - depends on the price of each.
If you go
HERE you can download a program for RVers that will make the simple calculation for you. It allows you to input the heater efficiently. If you see a box wanting you to sign in just close it and do the download.
According to an email to me from engineering at Suburban Manufacturing, the efficiency of RV furnaces ranges from 70 to 80 percent. Water heaters are 70 percent efficient on gas.
Note: Because Microsoft in their infinite wisdom didn't include the software to support the older help file formats, for Windows Vista and higher you will need to download and install a Microsoft program that allows you to use help files with the "hlp" extension for whatever operating system you are using - if you want to use the help file (not really necessary to use the program)..
You can download it here
Microsoft Knowledge Base. Scroll down to select your operating system.That doesn't work for Win10 however. Microsoft has not produced a patch for Win10.
The program is so simple that a help file isn't really needed. It just shows the info used to write the program .