Elderly Ford Cargo horsebox leans to drivers side?

Hi
My 1985 Ford Cargo leans lower on the driver's side. It has always done this but maybe is a little worse recently?(had a comment from a bystander when parked recently, which haven't had before) Has fuel tank on driver's side (not usually full as tooooo expensive!) and large water tank on other side. Water tank has been empty this year because some ...idiot... borrowed lorry and filled water tank with diesel! Just wonder if this lack of water weight is enough to make leaning worse, or if I have a problem with springs or something else technical - it drives fine, and passed plating in December.
Many thanks in advance,

Angie K
June 2010