Gravity feed is slow
your idea can work. In fact, my truck bed is a few inches above the Airstream fill point when both are on level ground. However, its a very slow siphon process. The pump really speeds things up. And, when the truck is below the Airsteam, I'm outtaluck.
Another road-story: our favorite water source in CO (Steamboat Lake State Park) had a pipe break and no water was available for a week. They brought in a large water tank (several thousand gallons) with a garden hose attachment. Unfortunately, the tank was sitting on the ground, and as the water level dropped, it became impossible get water into a trailer. Worse, such gravity feed won't fill the bladder because of the back-pressure that builds (also the resistance from the filter).
The SOB ahead of me took 30 minutes of trickle-time and was still only half-full.
Since I had the pump, I just inserted it into the line, started up the pump. Topped off the guy ahead of me (not because I'm a good samaritan, but I was tired of waiting!) and filled my tank in no time.
Point being: having a pump is a good precaution if your water souce is not always reliable.
Hey, its the only way to be sure!