Only two possible solutions
Move the inlet fitting on the tank to a location on the centerline side of it...or install a loop. Since the head only discharges into the holding tank, it doesn't have to be a vented loop, just a 180 degree radius fitting (pvc...you should be able to find one at any plumbing supply) that's too high for tank contents to climb over. The best place for it is immediately after the toilet...mount it on the bulkhead above the toilet...aim the head discharge fitting straight up to it. I'm concerned about a couple of things, though: 1. that you're getting a bowlful of tank contents. A properly working joker will allow seepage, but even if you stayed on the same tack for several hours, continuously heeled over enough to send tank contents back toward the head, there shouldn't be more than an inch or two in the bowl (Or is an inch or two a "bowlful" to you?)...in an hour on the same tack, there should barely be enough to notice. 2. I'm not sure that what's running back is even coming from the tank...it's more likely to be waste left in the head discharge line. Otoh, if it only happens when the tank is above a certain level, that does point to the tank. And I'd figure out a way to get to your plumbing if I were you...'cuz sooner or later your're gonna have to replace the hose, and you'll need to know where it goes.
