What does this coolant line do?
 

I have a 1990 Winnebago Warrior RV built on a Ford E350 van.

There are, of course, two bypass lines that come off of the water pump and manifold. Normally, these would route to the heater core. On my RV, there are tees in these lines that also run back into the RV house cabin, to heat a heater back there as well as heat the water heater (this is called "Motor Aid" - it will give you hot water by using the engine heat to heat your water heater tank).

In addition, there is a thinner (3/8"?) line running off of a T that goes to a fitting on the bottom of the radiator near the drain cock.

Can anyone tell me the purpose of this line? It is difficult to find a replacement radiator with an extra fitting for this line.

Schematic:
Attachment 285189

Here is where it tees off of one of the bypass lines:

Attachment 285190

Here is where it terminates at the radiator:

Attachment 285191

This line is a metal hard line that runs down behind the alternator. It has permanently crimped-on rubber ends at the T and down by the radiator. This line makes me nervous as it is 28 years old and I would like to replace the rubber portions due to age but as they are permanently crimped onto the metal middle run I'm not sure what to do about it. I could replace it with plain rubber tubing and just carefully zip-tie it out of the way. But it makes it problematic for replacing the radiator as I can't find an aftermarket replacement with a fitting for it to go to.

Steve