Portugal in January/February

We have been motorhoming for many years and pre-retirement toured France extensively, if somewhat quickly in two week breaks. Now we are retired we are enjoying spending longer away and are thinking of heading to Portugal for a month or so in January/February. 

Part of the joy of motorhoming for us is flexibility. If we like somewhere we stay a bit longer, if not we move on. Consequently we never book a site until a couple of days before we move. We are not wedded to  Club sites and have used ACSI (and other independents) extensively. We don't know Portugal and Spain so I have a number of questions:

Are there any preferred routes from the North of Spain down to Portugal?

Any must see/avoid places?

Any recommended sites?

Am I right in thinking it is illegal to tow a car with an a-frame in Spain? No we don't want a caravan instead!

Has anybody had any difficulty touring without pre-booking sites?

When. starting off we load our fridge with fresh food and freezer with homemade meals. Are there any issues with this post-Brexit?  Has anybody actually had their fridge inspected, or are the stories just hearsay?

We are thinking of using the Plymouth/Santander crossing rather than driving through France. What are other peoples preference? We won't use Portsmouth because of the ULEZ cost!

Any other thoughts/advice/comments gratefully received.

Thanks in advance

Mike