I think what UK was thinking about was the possible need to fill the tank with a jug, back country style, when a pressurized water supply isn't available.
I am not a plumber either, but I think your tank needs a vent in order to let the air escape when you fill it, and to let air back in when you pump. If your tank is completely sealed up, it wont work. Do you have another vent in the tank? If not, you can always just poke a hole in the top, but water will splash out when you're driving.
It's good to have a drain, but the water will only drain to the level of your outlet. From the picture, it looks like if you open up the drain, it will only empty out a little more than half.
I have done some amateur engineering with water flow. I set up a hydroponic garden in my greenhouse and made everything from scratch. A lot of trial and error. One thing I learned is what UK was trying to say. You can't fight gravity, everything must flow down, unless it is powered.
I think a good solution would be to keep the tank oriented as you have it now, but make the white tube the vent (hook it to your outlet) and make the clear tube the supply line to the sink. That way you can just pump it dry if you want to drain it. Leave clear tube long (extra 5 ft or so), so that if you want to drain it without a pump, you just hang the clear tube over the side of the trailer and let it siphon. As long as the outlet/drain hose end is below the tank water level, it will pull the water up and out of the trailer.
Good plan to use a hand pump for the sink, though. Hope this is helpful!