hi dave, i have never needed to clean mine but if i were to this is what i would do.
empty all water, lay in garden hose in tank with as much pressure as poss, switch on yor pump and let the water run out from your own wfp hose onto garden or somewhere suitable, continue until it looks clean then drain off all tap water b4 filling again.
if it's smelly i think you may get away with a bit of g3 to wash out but you will have to rinse well after.