The North lot is prime ground as it is right in front of the stadium. However parking is limited, expensive (around $40 and more for RV) and inconvenient (can't start tailgating early). There are a few lots that accept tailgaters but you must be attentive to their rules. The best small lot I've found is on holgate across the street from the office supply store (nice staff and bathrooms). If you are looking for a budget friendly option street parking is usually free. Like many have said the area on first Ave is nice with lots of good tailgate action. There is also a good area near the railroad tracks just off holgate and 4th. There are several blocks with a similar environment to what you will find on first. Seattle is not a great tailgating location, but I hope this helps you find a good place to have a good time!
"Watch your step, look both ways, and change your passwords." -free safety advice from Earl Thomas