You need to go up to as high as possible to a vacuum (siphon) breaker, then down to your holding tank.

If the bottom of the holding tank isn't above water Levelle, then it won't drain completely. In that case, you really should pump it up. (Using a vacuum breaker) if you want to pump it out.

You need to make sure the tank you get can handle the pressure of being full and being thrown around where it's mounted;and remember that the hose fittings have to tale a bunch of pressure too -- especially any that are low. Your thru hull should be accessible too, just in case while that won't soled a blow-out of the tank, it will allow you to avoid sinking.

There are simple diagrams that show how to plumb these. I would advise sticking with them.