question

If you want to get the content of a tank you need to cast the Object to a FlowTank (something like: token.Tank.as( FlowTank ).content.)

But since you are looking at a group I think you will need a short for loop or run subflow to get the content of all tanks. I would change the assign labels to something like:

Object current = param(1);
treenode activity = param(2);
Token token = param(3);
Variant assignTo = param(4);
string labelName = param(5);
treenode processFlow = ownerobject(activity);

Group tanks = token.lb_Tank;
double content = 0;
for( int i = 1; i <= tanks.length; i++ )
{
content += tanks[ i ].as( FlowTank ).content;
}

return content;

Hope this helps