If you go
HERE you can download a small program that will make the simple calculation for you.
It allows you to enter the appliance efficiency.
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 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.
You can download it here
Microsoft Knowledge Base. Scroll down to select your operating system.