I have a 2001 36S DP but the air tanks and lanyards have always been similar on Freightliner chassis.  If you look at the attached illustration you will see the two air tanks and location of the lanyards.  The lower right illustration is shown from underneath looking up.  Two of the lanyards are located in the right front wheel well and one in the left front wheel well.  If you get under the coach you can easily see them.  Also, depending on the Owner's Manual or service manual it recommends draining every 12 months, six months, or every 12K miles.  It's so easy I drain them every month.  I hope this information helps you find your lanyards!