Washington State roundabouts usually have a paved ("mountable truck") apron (in a contrasting color/texture) that long vehicles are welcomed to use.
https://www.wsdot.wa.gov/publications/manuals/fulltext/M22-01/1320.pdfAccording to the state, using both lanes is acceptable practice
Multilane roundabouts have at least one entry or exit with two or more lanes and more than
one circulating lane. The operational practice for trucks negotiating roundabouts is to straddle
adjacent lanes.
https://www.wsdot.wa.gov/Safety/roundabouts/default.htm