You didn't say which Designer 5'er you have, but mine are:
Fresh water tank on the outside just behind the right rear tire.
The others are in the basement on the curb side. Open the curb side door and you will see two small openings secured by screws. One is to the left side (rear) as you stand looking into the basement and the other is on the right (forward). Both are down right at the basement floor. The one to the rear has three drains as I recall and the forward one is for the washer/dryer and there are two in this area. If there aren't three in the rear access area, then just drain all that are present. They are all right there in the same spot.
Don't forget to drain the water heater also.
2006 GMC 2500HD CCSB 4x4 Duramax/Allison, Titan 52 gallon fuel tank, Prodigy Controller, B&W Companion Hitch
2010 Jayco Designer 35RLTS, Cummins/Onan RV QG 5500 EVAP, Progressive Industries EMS-PT50C, TST Systems 507 TPMS, RV Flex Armor Roof