Besides the 2 low point drains shown similar to the photo in Lou's post, there is probably a drain for your fresh water tank.
If you are going to blow the lines, the 2 low point drains should probably be capped during that process. That allows you to pressurize the system, and then blow through each valve at each faucet. With the caps removed, all of the pressure will go out the low point drains.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263