Band-aid vs. permanent fix
A new joker valve will BLOCK water coming back into the bowl, but only for a short time, till it becomes a little worn....it's just a band-aid that won't solve the real problem: water and/or waste left sitting in a head discharge hose.
Water runs downhill, so the REAL cure is a) use the dry mode to move it all the way to the tank..any manual toilet that's working anywhere near factory spec can move bowl contents at least 6 linear feet in the dry mode or 4 vertical feet. Or, if the run is too long for that to work, b) replumb the head discharge line to put a loop--doesn't have to be a vented loop, just a loop--immediately after the toilet so that it's only necessary to pump long enough in the dry mode to clear the top of the loop...gravity will get it to the tank.
But I don't think water left in the head discharge line is your only problem. You said, "... during the process of rinsing the holding tank, the entire toilet bowl filled up with backflow. It almost overflowed. So, obviously, there was no pressure on the holding tank from a clogged vent, as the tank backflowed while the main tube was completely open."
Actually there WOULD be pressure from a blocked vent while filling the tank with water, because a) water is filling the pumpout hose, leaving little or no room for any air to get into the tank through it...and b), once the water in the tank is deep enough to cover the tank discharge fitting, no air can get in through that line because the pumpout hose is at the bottom of the tank, covered with liquid that prevents air from getting into the tank. If the bowl filled while rinsing out the tank, it could only have done so once the water in the tank was above the the tank inlet fitting...and if it wasn't pouring out the vent too--not just trickling out, but pouring out, the vent IS blocked.
Something none of you have thought of: it may NOT be BACKflow. If the water filling the bowl is clean, and if there's no vented loop in the head intake, the intake may be the source of the water caused by a failed wet/dry cam--a common problem in Jabsco toilets, or the toilet left in in the wet mode.
But if it IS backflow, replacing your toilet with a PH II--or any other toilet--will not solve your problem, 'cuz water will still run downhill, joker valves wear at about the same rate in all toilets, and/or the tank vent may still be blocked.
So if you want to solve the problem permanently when you install the new toilet, replumb the head discharge line to eliminate as much of any uphill run to the tank as possible, install a vented loop in the intake as shown in the installation instructions for all manual toilets...and learn to use the dry mode to clear all the water out the head discharge line.
And btw...I'm not a fan of the new Jabsco "Twist 'n' Lock" toilet...'cuz no one who has one will ever replace the joker valve 'cuz they don't know what its real function is...no backflow, so they forget it's there. And, as many people as there are who don't know any better than to keep pumping a toilet against the increasing backpressure from a blocked tank vent, something HAS to let go in the system when they lock that pump down after building up enough pressure. If they're lucky, the next person to use the toilet will only get a face full of sewage from the eruption in the toilet when they release the handle or from the deck pumpout fitting when they loosen the cap.
IMO, there has to be a limit on the number things mfrs do to try to save fools from themselves...and that toilet is past that limit.
