Originally Posted by mouth012006
I am not sure how to go about cutting out the section that needs to be done.....the venting will need to be done for the top section of the hot water heater.....
A 4" hole saw is a great tool, just be careful because the torque of a misaligned holesaw can be awesome. Make sure you're putting the hole in an area that doesn't have a stringer or a rib inside. You can tell by the rows of rivets (usually). Also, think about where wiring might be--all circuits come down from the center of the ceiling, so generally speaking
if there is no outlet below where you're cutting, you are OK. A hole saw is safer than a saber saw in this respect, if you carefully prevent the saw from diving inside the wall as the disk is cut free.
A saber saw works quite well on Airstream aluminum, but you need to tape the skin liberally to avoid scratches from the foot. Also, the blade, when fully extended, is probably longer than the depth of the shell, so you'll hit the other skin if you don't devise a standoff of some sort, like a piece of 3/4" wood under the foot of the saw.
You might also look for a hand-nibbler at Harbor Freight, which needs a 3/8" hole to start.
BTW, have you thought about providing a combustion air inlet so that the heater doesn't draw cold air in through the door and window seals? The down side is, such an inlet provides another path for outside air infiltration when the heater isn't working.