Anyone tried this quick-fix mini solar panel?

At £27 it's quite expensive for 5W. There is no need for a regulator on such a low output panel so, yes you can just connect it straight to the battery either by a cigarette lighter socket or directly onto the battery with crocodile clips. If you intended to use this during the winter you would probably have to put the panel out in the open air (rather than inside), the tinting of the windows will reduce the light falling on the panel. In the winter there is little power in the sun between October and the end of Feb. A 5W panel will only trickle charge a battery in the height of summer. If I were you, in the winter I would remove the battery and put it on charge at home with a 'smart' charger - the type sometimes on sale in Lidl and Aldi. These are something like a Ctek charger but at about £20 as opposed to £75. This type of charger is 'fit and forget' unless you have a power cut, then you have to switch it back on again. Removing the battery also gives you the opportunity to check the electrolyte level (assuming it's a 'wet cell battery').

Hope this helps.

 IMO, you were been given somewhat uninformed advice.

To "enable" any more modern caravan's 12 volt sockets to be connected to the van's battery, the 12 volt system needs to be switched on. The issue with that is all the sundry power drains the system features will be sapping power 24 x 7 hours; with every chance of flattening your battery faster than a 5 Watt solar panel could ever replace that drained energy.

So, point one, you would need to connect to the battery itself, ideally directly, and ideally with any other connection removed so there is no drain sapping away there. Dedicated solar input socket where provided connect directly to the battery, they have to.

Secondly, unless the battery is already all but "shot" it will hold a healthy charge for some months; therefore fully charge it and disconnect it and you ought not need do anything for a couple of months.

Thirdly, 5 Watts is pathetically low and at £27 a rip off; there are more cost effective 5 Watt solutions out there LINK

I would suggest a 10 plus Watt makes more sense. More begins to make greater sense, but only if actually needed, feeding alarms etc.

Plus, if the panel is left inside so the solar energy has to punch through the plastic window before striking the panel then at best the panel can't achieve its already pathetically low rating. Add to that is any window facing the right way to gather much daytime sun, void of shadows, etc as the sun tacks across the sky?  [put it out flat on the roof for some hope]

Basically, I can't see you need it; as you were told to wire it, was fundamentally flawed, and the panel size too small to achieve anything worthwhile.

Agree that 5w panel in that configuration is largely a waste of time & money. However, it looks like this post has been stuck in a CMC black hole since 16 Feb, so op more than likely given up and moved on?