Find a hole and fill it...
Look around your unit, there has got to be a hole somewhere behind a counter or under a seat, it does not matter if it is above or below the rest of the system, it only matters that the fill pipe is at the BOTTOM.
Once you have found the hole, next look for a tank to fit, and if a tank cannot be found, make one.
A local irrigation supply or farm service may have remnants of large PVC pipe that along with a couple end caps a tank can be made.
The diameter of the pipe to fit one dimension and the length to fit the other, no reasonable limits to the size.
Thread and tap one cap or adapt, when you tell them what you are upto they may see it as a good idea and help.