Geoff,
As Jon said, most of us have separate policies for the umbrella policy. A policy of $1Mil - $2 Mil is common. I just reduced mine to $1mil and changed my policy to RLI Insurance Co. with an annual premium of $397. That premium is based on the number of vehicles and properties they have to cover. Mine covers the bus, motorcycle, car, truck, SUV, conversion van and 3 small houses. (You could easily place all 3 of my houses inside the Keller or Foster Compound.) But at the same time, I am highly Blessed and have nothing to complain about.