18 or 24" power jack?

It all depends on how your RV is built. Over the axles build (no wheel wells) would require the longer jack but most "around" the axle units (with wheel wells) can get by with the shorter one. With a longer jack you MAY end up with less ground clearance or taller jack piller which can interfere with tailgate opening.

Either will work although the longer shaft will make the process easier in most cases.

If you are going to spend the money on a Barker, get the 24". I just nailed 4 2x6's together, or you could use a couple 4x4" post pieces, etc. Not a big deal, but I spent $100 on on electric tongue jack.

You want to make sure the jack you get is long enough to lift the camper high enough that it will completely clear the ball on the tow vehicle, otherwise you'll be putting stacks of lumber under the jack so it will rise high enough.

If you are ever parked on a slight incline, a spot that has a dip in the wrong direction, or uneven ground, you may find the shorter jack does not have enough lift to clear.

Also, you need to make sure you have enough lift so you can put your distribution bars on. The higher the bumper of the truck raises, the easier the bars are to put on, unless you simply enjoy using a cheater bar to wedge those bars, or chains in (depending upon what system you're using).

Go with the longer one, you won't be disappointed.