You won't find the anti-siphon loop in the Jabsco parts
kit as it is not a part of the toilet itself. Placing a vented loop between toilet discharge and the holding tank stops swage from back siphoning from the tank back into the toilet, should the level in the tank get high enough.
Even wtih an anti-siphon loop, it is possible for the water contained within the hose between the pump and the top of the anti-siphon loop to slowly flow back through the pump and into the bowl. As mentioned this is likely because the one way Joker Valve is not getting a proper seal. It could be the valve is old or it could be there is a bit of toilet paper or something else preventing it from making a complete seal.
Given the cost of a rebuilt kit, I also think it's good just to carry a complete spare pump. It's much easier to replace the whole thing that trouble shoot all the parts. You can then trouble shoot the old pump at your leisure and replace what ever is necessary so it becomes your spare.
Most toilets I've had loose the joker valve seal fairly easily. It's a good reason to make sure your anti-siphon loop is not higher than needed. The less volume, the less that can drain back into the bowl. When I'm in a discharge area, I also tell people to pump quite a bit, to ensure all sewage has been flushed from the hose. Not only does this prevent sewage from backflowing, but means it is much less nasty to trouble shoot the pump if necesssary.