Usually most insurance companies want all coverage through them if they are going to issue an umbrella policy.
When we did our 8 month full time on the road trip I called my insurance company and had them issue an umbrella policy. I figured if I jackknifed the trailer across 4 lanes of traffic and caused havoc I would be covered.
As with insurance most of the time, we had an uneventful trip and I did not renew when I got home.
On this board you will get many recommendations for insurance and as many say those are bad companies. Myself, I will never insure with Geico, due to an accident that wasn't my fault and the other party had Geico and it was a nightmare to deal with them.
Do your own research.