No idea, I've never used mine (same size/heat source) either.
However, I read in this article that on most
heaters, just running the pilot (assuming it has one) is usually enough to heat the
water overnight for morning usage.
It also has a lot of other useful RV plumbing info.
When you figure out how long it takes, could you let us know? We're probably going to try
our heater this year (sometime). If I ever get the gunk cleaned out of it. I don't think it's
ever been used or cleaned/flushed. The anode is completely gone.