I can't speak to its use, because mine was damaged when I bought it (some of the PEX tubing leading to it was broken), and since an RV-ing buddy said his never really worked all that well (non-Chinook RV but same flusher style), I decided not to repair it but just to remove it.
So why am I even responding?

Well, just to say that if you haven't taken a look yet, and presuming yours is close to my '99 (sounds the same from what you have described), you can see/get to all of the PEX tubing that leads from the outside fitting to the black tank. It's mostly behind the driver's side "wall" of the outside compartment that is just ahead of the stock spare tire location. You can remove that wall with just a screwdriver, and put it back easily as well. The PEX tubing that isn't in that area is underneath the rig where you can also see/get at it. Then it goes into the aft sidewall of the black tank.
If/when you are in that sidewall area, you will see all kinds of interesting things: Much of the grey and black drain/vent piping (1-1/2" ABS); quite a bit of wiring (heading over to the other side of the rig via over the back door - taillights; etc.); the 1-1/2" drain piping as it goes into the tops of the black and grey tanks (through the floor), supply tubing to the toilet (1/2" PEX); and lots of empty space alongside the shower basically begging for insulation. Well worth a look!
Side note is that when I did this early on in my ownership (went into the wall area), I then saw how much wasted space there was in the "tire locker" forward of the forward wall (i.e. back side of the bathroom sink wall). I thus put the side walls back, but not the back (forward actually) wall. Instead I put some coated insulation board up against the backside of the bathroom sink wall (where the removed wall used to be further out), and then built a little wooden "box" (two sided) to cover the ABS drain piping and protect it. As a result my "tire locker" is probably 30% larger, and I can get to the little "tunnel" under the shower (this will make sense when you see it), which is where I store my spare belts and torque wrench. Naturally I saved that "wall" for future just in case (and/or if you still have the stock interior and have any water damaged panels this would make matching repair stock).
I noticed that in the newest Chinook 21-ers (say 2003 and up) they re-designed that forward wall to take better advantage of the space.
BG
PS: Meant to add that if you haven't worked with PEX before, it's super simple. Makes plumbing work go from a nightmare to like working with Tinker Toys. Basically you just cut it and mechanically join it. The cut needs to be straight, but an inexpensive cutter does the job. You can buy all this at a Home Depot/Lowe's etc. because it's used in houses as well now. To join it you can use "expensive" options like brass, push together "Shark Bite" fittings (but not that expensive in the numbers we need, and very easy to work with), medium priced options like plastic push together fittings (various brands), or the least expensive which are like Oeticker clamps (sort of like hose clamps in a way) that you clamp closed with a special tool.
The latter are the cheapest, and take the least space (which can be key in certain areas), although you have to be able to get the tool in. You can see these, plus the plastic push-fit (on valves) in the stock Chinook setup. After working with a few of the different ones, I like the Shark Bites -- although you'd go broke doing a whole big house with them, we don't have to care about that.
Last note is that the push together fittings can be removed/replaced. The Oetiker style clamps you cut off and then re-do if you want to remove/replace.
Any of them are like child's play compared to other plumbing types, at least IMO, a non-plumber.
PPS: There is an older style of similar tubing ("PB" if I remember correctly) that ended up not being as viable in the long run (for RV's or houses), so is no longer used. This would not apply to your rig, and even my '99 has PEX from Chinook; but I mention it as older rigs (mid-90's or older) might have PB. My previous 1997 camper van did have PB. It's typically dark grey. The connection methods look similar but the push-on fittings are not exactly the same inside. However there are special joiners at the stores so one can transition from that to PEX when doing new work.