Clay, you are thinking about personal effects coverage, not an umbrella policy. Personal effects coverage is part of the RV insurance policy, and for fulltimers, may need to be increased (we, for example, have $20,000 in replacement cost personal effects coverage). An umbrella policy, on the other hand, is a separate policy and requires underlying insurance...the underlying insurance pays first, and the umbrella policy takes up any slack. Our umbrella policy requires that we keep certain levels of coverage on our underlying insurance.
To the OP: Not all insurance companies offer fulltimers RV insurance, which combines "RV insurance" with "homeowners insurance." If you haven't already found an insurance company that offers fulltimers insurance, I'd recommend contacting several insurance agencies that specialize in RV insurance...ones like Miller's RV Insurance, Gilbert RV Insurance, etc. (just Google "fulltimers RV insurance"). Or you can call some companies such as Progressive, GMAC, and Foremost directly...just be sure you're talking to their RV insurance department and not their auto department.