Yes. An RV with 2 or more people in it is allowed in the HOV lane where 2 or more people in HOV lanes are required. According to the WSDOT website, there are two areas where HOV passenger requirements are different. They are SR 520, west of I-405. This is the only place on the HOV system where three or more people are required. The 3+ occupancy requirement on this segment has to do with safety concerns.

Also, there's the I-90 Reversible Express Lanes, west of Island Crest Way. Single occupant vehicles traveling between Seattle and Mercer Island and vise versa are allowed to use the I-90 HOV Reversible Express Lanes.

If size of the vehicle is the concern, RV's are allowed. The WSDOT website says trucks are limited to 10,000 lbs. GVW. Towing is allowed in the HOV lanes as long as an adequate speed can be maintained and the combined weight is 10,000 lbs. or less. There is no weight limit for recreational vehicles.