Mike,

There's a few good RF meters out there on the market, but do you really want to spend $1-2K just to prove your point?

If the problem is apparent at the street, why not hook a direct line from the street to the television and show the cable company that the problem's not inside the house?

There's no point in losing sleep while trying to figure out ways of making the cable installers understand. You're dealing with the lowest common denominator here.