I use Command Strips by 3M. You can get the refill strips for their hooks separate from the hooks themselves and use them like double face tape to attach the mounting plates or the device/item themselves directly to the wall without having to worry about finding studs or having screws pull out of the thin luan that they use under the wall paper. We don't use towel bars for our towels, but, use the large Command Hooks to hang them on...takes up less space and we can "hang" more towels if needed than you could with a regular bar. The Command hooks even come in "brushed nickel" that matches the hardware on the cabinets pretty well.
Have mounted napkin holders, toilet paper holders, coaster containers, clocks, hooks for the bicycle helmets, pot holders, etc. with them, have even used their large "heavy duty hooks" themselves as brackets in the forward storage bay for hanging the stabilizer wrench, hot dog skewers, awning rod, etc. They hold well, remove easily and in almost 30,000 miles of travel, on some pretty nasty roads, have never had one fail; must follow the directions though...mount the strips and then wait an hour before adding any load to them...that allows for the "setting" of the adhesive.
Like Post-Its, 3M has a gold mine in those silly strips. Go on easily, hold tightly and remove cleanly. The only draw back is the little tab that you have to keep "in the clear" so that you can pull them to remove the strip. Think ahead, though, and the tab can be placed so that it isn't normally visible. For a towel bar, I would use 4, 2 on each side's mounting plate...the heavy duty strips are rated at 5 lbs each if memory serves me correctly...that would be 20 lbs of support for the whole bar; more than enough for a couple of towels, even wet ones.
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)