The only way to learn is to do it! My dear wife always packs like we are going to Death Valley when we are really going to a town of 15,000. I think once you do a trip or two you will find your balance and be more comfortable with the whole situation.
Like Gizmos Mom and others, I make a check list and I start putting stuff I want to be sure and not forget in a big cardboard box in the garage. Before starting our trip and every morning when we have spent the night and are taking off again, I treat our rig like it was an airplane. I do a walk around visual inspection of all compartments, mirrors, tires, etc., making sure everything is disconnected and stored. Once you do it a time or two it takes less than two minutes, including climbing up the rear ladder and doing a visual on the roof. If the antenna is up, you will spot it for sure. Like others have said, it is a good idea to do something to make sure you don't start out with the steps out (most retract automatically when the engine starts), the antenna is up, a hose is still connected, etc. ALSO, I use a checklist. ALSO, I use a stickit note over the speedometer to remind me of anything I think of at some point I might forget to do.
After a few trips, all this becomes pretty routine and automatic. The most important part is this: remember this is a hobby, not a job and we are all traveling for fun.
PSW
2013 Phoenix Cruiser 2350
2014 Jeep Cherokee behind it
and a 2007 Roadtrek 210P for touring