This issue had been bothering me for a bit also. I just checked the opening on my trailer where I had pulled out the rusted decaying carcass of the original water heater, and it is already set for a 10 gallon. My manual also says 10 gallon for the International, which my trailer is. I will be putting back a new 10 gallon to fit the opening.
I'm considering the electric 110v and LP. Is it advisable to create a separate plug for the water heater and just plug it in to a standard outlet at a campsite, vs. hooking it in to my panel box and taxing my power supply. (i.e., running microwave, HW heater and other 110v at the same time.) I heard this sometimes suggested for adding a second AC unit. My circuit panel is right above the water heater in the linen closet, but I was thinking since I have the toilet out, and adjacent cabinetry, it would be easy to just create and run a separate line, and not have to worry about it.