Hi Jim,
You may have the same problem as I had when I first had my van, I tried to empty the tank but had no luck, mine is the same as yours with the yellow leaver when it is in the up possition it is open and should drain.
What I found to be the problem was a red bung (plug) in the outlet tube underneath the caravan which stopped the water coming out, have a look under the van near to where the hot water tank is and you will see the tube there.
On my van I have two outlet tubes one for the hot water tank and one for the cold water system that can be drained as well both had the red bungs in them.
Whatever you do don't loose the bungs, the cold water system one needs to go back in before you refill the system.
I think they are placed in the outlet tubes at time of manufacture and should be taken out by the caravan retailer prior to selling.
Hope this helps
Graham (Ponty)