You would be hard pressed to fill the 21 gal gray tank on the 19' unless you literally emptied the 23 gallon fresh tank into it, which you'd be pressed to do if you used the head at all. If you are on city water hookups, then it would be very easy to fill a 21 gallon gray tank...heck it would be easy to fill any tank, even my 39 gallon gray tank if I showered in it daily with city water hookups. If I were on tanked fresh water, again, I would have to put every gallon from the fresh to the gray to fill it.
Now the black tank, even in my 25' SS, I have the same black tank as in the 19' (18 gal). For two folks, you could last well over a week and not fill it. I had three adults in my trailer and we didn't fill the black or the gray in a bit over a week.
The combo tank found in the current 16' unit, IMHO could be an issue, at least for me. In some remote campgrounds you can dump gray water if you have a gray tank, but if you have a combo tank, and it's that small, you could fill that 21 gallon tank pretty darn quickly given that all the contents of the 23 gallon fresh tank, plus the extras from the bathroom deposits made would most likely go to that one tank.
When I boondock, I bring one of those 5 gallon portable coleman jugs that I keep outside for washing hands and washing some dishware. I do this for a few reasons. First, it helps conserve my fresh water tank level. Second it keeps the tanks less full if you can't dump the gray water (which some campgrouds don't allow). Third, it stops a lot of in and out of the trailer letting bugs and worse into the RV.
In the end, I was very happy with the 19' Bambis tanks. Fresh water was a bit small as was the interior space for 2 adults with a 65lb dog. I love the 25er and the larger tanks are a bonus as is the added real estate.