Official WA State Code Here. According to that, only trucks with GVW of over 10K lbs are restricted. Most states don't include pickup trucks and light trucks as "trucks." And, there is a specific allowance for RVs. This can be corroborated in various articles online ... I read one that featured a Q&A with a state trooper, for example.
So, legally, it looks OK.
The question is one of etiquette. That ends up being very personal and local (i.e., what's happening in the flow of traffic at that location at that time).
I'd be loathe to hold up traffic and would use HOV very sparingly. But, that's your call.
Good luck.