Also recommending I 5 through Seattle. On a Sunday it shouldn't bad. I would avoid 405. it's narrower and has more on/off and sunday shoppers - more stop and start traffic.
Gas up north of Seattle (consider Tulalip Casino / reservation around marysville) and try to get past Tacoma before you pull off the road to avoid the worst congestion. The Tulalip casino exits have easy on/off access gas, food and there is a Cabelas at that exit.
Regarding the Car pool lanes, you can tow in them but must maintain speed. They tend to be on the left side and often have jersey barriers on both sides. You may want to avoid them for this reason.
More info here: WSDOT - High Occupancy Vehicle (HOV) Lanes
Who can use freeway HOV lanes?
Drivers can use HOV lanes whenever there are two or more people (including the driver) in the vehicle. The definition of "people" is not limited by age or licensing status. Hybrid and other green vehicles are required to have the same number of occupants as other vehicles in the HOV lanes. Trucks are limited to 10,000 pounds gross vehicle weight, but there is no weight limit for recreational vehicles or buses. Towing is allowed in the HOV lanes as long as an adequate speed can be maintained and the combined weight does not exceed 10k.
Buses, emergency vehicles, and motorcycles can use freeway HOV lanes, regardless of how many people are on board.
Who cannot use freeway HOV lanes?
- People driving alone, except as noted above.
- Solo drivers who are passing another vehicle in the adjacent general purpose lane.
- Solo drivers in hybrid or other green vehicles.
- Trucks which weight more than 10,000 pounds gross vehicle weight.
- Vehicles pulling a trailer when the total weight of both vehicles is more than 10,000 pounds gross vehicle weight, or when the combination is unable to keep up with the flow of traffic in the HOV lane.