If you haven't already started looking for something else, start lining up interviews now.
IMHO you should be working as a consultant, and charging an appropriately high fee for such. Your intuition is probably right; they want to suck as much knowledge out of you as they can, then leave you unemployed while they implement all your suggestions.
Intellectual property created while under the employ of a company belongs to the company, and that includes a plan for fixing their SEO problems. If you make something for them, it's theirs... and those plans aren't transferable so I can't imagine what you'd gain by holding out on them. There is NO LACK of companies wanting SEO advice.
You might feel that you're being exploited. To protect your self-esteem (and further your career), you should just see this as a contract position, a step on your journey; get what you can from it, offer what they need and - if they decide not to hire you - leave with a smile and warm wishes. Don't hold a grudge, keep in touch with their webmasters and HR staff, and let them spread through word of mouth what an awesome SEO expert you are, such a great and knowledgeable chap who gave them magic advice during your short stay; isn't it too bad they couldn't afford to have you on staff permanently! yadda yadda yadda.
Plant a good seed and it may bear fruit later in your career.
Then next time they want to "hire" you, inform them of your consultancy rates (which should be significantly higher than an employment salary)