First, the chance of the fuel cap I never use being magically loose doesn't exist. Second, my 2003 Sig manual says NEVER fuel both sides at once, it will spill. I thought my Guage was bad first few times out. I filled one day and the next stop my computer said I had over a quarter of a tank. I then ran out of fuel. That is when I started tilting. Now I have a real 150 gal tank. I have never lost a drop from the vent or either cap. I would not fill it that way and park in the sun. I doubt if expansion would run fuel out the vent, but I don't fuel right before arriving. Look at the tank design. Without standing the rv on its side, there is always going to be a large air space with no fuel. The tank is a 150. That allows for a percentage of expansion room. I don't go crazy tilting, I did the first time and it took 30 extra gallons, probably putting me closer to 160, (I had run out and got approx 30 gal) I tilt down on pass side or up on drivers, not both. By all calculations since, I am filling close to 150. That is including the extra 20 or so the tilt adds. Putting 150 gallons in a 150 tank is a no brainer, it was designed to hold that and be legal.