First off Grid trip

What a dreadful thing to do!  I would not tolerate that on a pitch near me.

Generators and ICE engines aren't the most environmentally friendly way of charging a battery, not to mention several hours of racket for your neighbours.

if folk want to be off grid (as an ongoing camping choice) and require the battery to be topped up throughout their stay, then a solar system seems the best way to go.

the savings on lower fees at CL and THS type sites will rack up at £10 or more a night and soon pay for that install.

furthermore, if this becomes a regular thing then the (typically Calor) gas supply will soon become a noticeable expense and investigating a refillable system like Safe fill (for caravaners) will be the next improvement. 
again, using gas at less than half price of Calor will also bring a smile to your face.👍

And with smart alternators on most modern cars it quite possibly would not work?

Back packers maybe but not most campers.  We tented in Europe through the 80’s and 90’s always off grid. However, you could buy blocks of ice, or get your cold packs frozen, either on site or in a nearby shop. Those in a decent cold box preserved food equally as well as a domestic fridge.

There are sites which offer battery charging, many CLs do.

We got our solar panel from Photonic Universe https://www.photonicuniverse.com/

Honestly, 60w (ish) is ample if it’s mainly for lights and charging devices. You would need a bigger one if you have a TV but don’t forget the bigger they are the heavier they are. 

There is a tremendous difference in performance between a free standing panel and one mounted on the roof.   The freestanding panel will be much more effective when deployed because of the angle it is deployed.   A 60W panel would probably be just enough if freestanding.   If roof mounted I would go bigger because the difference in cost is going to be fairly minimal.    

agree with Boff re performance...facing the sun is best...but remember a stand alone panel needs to be turned throughout the day (the line of sun moves) and also needs to be made secure (locks/cables) or someone will have it...

Is there such a thing as a ‘sun tracker’ on a small enough scale for LV’s or am I guilty of way too much SciFi watching BB?🤷🏻‍♂️😬

Ideal is obviously south, rule of thumb for angle is latitude + 15 degrees in winter, latitude - 15 degrees in summer. There’s not a huge difference between efficiency on flat mounted -v- 30 degree mounted panel in summer .