Hopefully you realize that the 120V option allows 120VAC operation in
addition to LP operation; you'll always have the choice to heat water with LP (or
both concurrently for quicker heating when you have both energy sources available).
Having said that, you can download the manual for the water heater in the files section of the forum at
https://www.escapeforum.org/forums/downloads.php?do=file&id=80
There you'll find on page 6 that the electric heating element is rated 1440W @ 120VAC = 12A
FWIW I'm with David - I'd go for the 2-way (LP/120VAC) option but wouldn't run it in the electric mode unless I had shore power (and then I definitely would use the shore power). That flexibility has served me well in my Casita for decades.