The most common answer to this question is micro-fiber camping towels. These are sold in some camping stores and online. We looked in two local large chain stores, one a camping store and the other an outdoor sports store. Everywhere we looked we saw the same small micro-fiber towels. There was certainly nothing very useful if one took a shower or went swimming. What are considered hand towels in what we saw were more the size of wash cloths.
The "Sham-Wow", as seen on TV, was suggested. We had one of these that we had purchased at a farmer's market for a dollar. I was told that to make this stiff towel feel soft to the skin, soak it in water and let it dry. We tried that and what we got was a wrinkled Sham-wow that was not really soft. I tried at home using it to dry my hair. It felt terrible to use. That idea was put aside. I should say that these are good in the Roadtrek for drying the floor after you have taken a shower in the aisle. While most of the water does goes down the drain in the floor, the floor does remain wet and these will do a good job at drying the floor - and drying the shower curtain.
Micro-fiber towels are basically the same no matter what their purpose. You can find them in the automotive departments to use to wash and dry your car. You can find them in the housewares departments to use as kitchen towels. They are all pretty much the same - and a bit heavier than the special "camping" towels that are sold for a lot more money - plus they come in a variety of sizes.
Our first purchase of towels were micro-fiber towels found in Wal-mart's auto department. There we found large bath size towels and medium sized hand towels. Of course, they were not called bath towels or hand towels. They are all purposed there for your car, but the sizes were close. These towels are soft to the touch, easy on the skin, absorb water and dry relatively quickly. The drying time is the most important part. On our first trips, these were the towels that we used.
In the Roadtrek there are three towel rods. One on the bathroom door, another on the large kitchen cabinet below the sink, and the third is on the clothes closet behind the driver's seat. As you can see none of these towel rods is very long and only the rod on the bathroom door (above) has any room for a thick towel to fit through. Micro-fiber towels tend to be thinner than the towels that you would get for your home bathroom so they don't have a problem fitting onto the rod, but there is not very much room across to put more than one or two towels depending upon their size. These three rods are all you have in the Roadtrek for drying anything on. You can hang a clothes line across the aisle, but no matter where you put this, it is going to be in the way. The best idea for this is behind the driver and passenger seat across the width of the van, but if this is there and you want to sit in the front seats to watch TV, you are not going to see anything but whatever is hanging from the line. It is a good place to dry things after you go to bed.
A short while after we had the micro-fiber towels, I read on an RV forum about quick drying, home bathroom towels that are sold in J.C. Penney. The towels have the label. "Linden Street" and the sign in the store says "Quick-dri". These are just like the towels that one uses at home, but they dry much faster. The come in the standard home towel sizes and there is a wide variety of colors including colors with stripes. The towels sell for $3 to $20 depending upon size. They are all cotton. Here is a link to the towels on J.C. Penney's website, but you can find them in your local Penney's store in the towel department. We tried a few of these first and they are great. They feel as comfortable as any home bath towel and they really do dry quickly. According to the store they dry 33% faster than regular towels. I am not paid by anyone to tell you about these, but they really are good for RV travel. We now use these for all hand and body use and the micro-fiber towels are now used for cleanup and drying dishes. Of course, we carry paper towels and when not needed, they are stored inside the microwave.