Mine turned out to be a bad water pump seal. If large quantities of coolant is lost, that'll be something obvious, but if small quantities are lost at regular intervals, it's harder to track down, as it's probably only happening under full pressure. It might be a water pump seal, or even a slightly loose hose clamp that doesn't show itself except under pressure, and it might not leak more than a few drops a day to where it nearly or completely evaporates before dropping to the ground. Splatters or areas of white will be an indicator. Might be a pinhole in a hose, or a pinhole seam leak in the reservoir. Might be a head gasket. The dealer can do a pressure test and check for leaks, and if none are found then they can do a hydrocarbon test of the fluid (indicates head gasket leak) and test for coolant in the engine oil (head gasket leak).
In any case it's not supposed to be happening... the small amounts over regular intervals, so it should be checked out. MIght be something minor, might me something major that is still minor at this point.
Also, the antifreeze is also a coolant (but 100% coolant is a particularly bad heat conductor, thus, it won't cool very well without the addition of water, which is a very good heat conductor), and it's also an anti-corrosive for these aluminum cooling systems. I forget what the deal is exactly, but if you don't mix it 50/50 the anti-corrosive properties are out the window, as they are organic corrosive inhibitors and need the water to make them effective.
Distilled water is not a bad idea, but it's kind of overkill with these aluminum cooling systems and the coolant, provided you flush and renew the coolant per the maintenance schedule, which is every 160,000 miles. Or sooner. My Sprinter tech recommends 100,000, or somewhere between 100,000 and 150,000. If you let it go and don't flush and renew, then distilled water or not, the organic corrosion inhibitors have broken and down to become useless, and corrosion has already begun to eat away at the insides of the cooling system (early signs of which includes small amounts of coolant lost over regular intervals, though of course a pinhole leak in a hose or a seal going bad will have the same results). If your tap water is hard, then don't use it. Hard water will shorten the useful life of the anti corrosives in the coolant, since they have to work too hard from the get-go just to break down the minerals in the hard water. If you have hard tap water, just use bottled water, but not the one gallon jug el cheapo bottled water, as that may very well be unfiltered tap water from a bathtub in Pittsburgh. Use bottled water that has been filtered, like Dysanti (or whatever Coca Cola calls it, or is that one Pepsi?). Pep Boys and other places has "mineral free" radiator water you can buy (don't drink it, tho). It's not distilled, but it's filtered. Or, just use distilled.
I changed mine at 100,000 (well, had the dealer do it) and he used bottled, filterd water. Same thing at 200,000. Also had it changed again, unwillingly, a week later when the water pump went south. My regular mechanic did the water pump, and he also used bottled, filtered water, and not tap water.
But the key is to keep that coolant flushed regularly, and often, like at 100,000 miles or so.
