The transformer in the BIP (the power unit) supplies the lighting circuit with 12V.

In the absence of a battery and the different electrical configuration of a power unit which is designed for 'off-grid' (autonomy) use, this output is often more than 12V (sometimes in the middle teens) and neither is it necessarily 12VDC since lights will work on either AC or DC.

Neither of these matter provided that you replace the existing lights (or the bulbs in them) with LEDs which will work off AC and DC, and which have a voltage operating range of, typically, 12 to 30V.

I've been down this road myself. Our 2008 Troll did not have the autonomy set-up from new (it was subsequently retro-fitted with a battery and the facility to charge it), and the transformer's 12V lighting circuit put out about 15VAC.

Ish.

This caused problems - mainly buzzing and flickering - when I first replaced a few of our bulbs with some tuppenny-ha'penny LEDs I had kicking about as an experiment until I realised what was going on.

Swapping those LEDs for decent ones which ran off AC as well as DC solved the problem.

Don't buy cheap ones. The likelihood is that they won't work properly, and if they do they probably won't last very long anyway.

There are loads of decent ones to choose from, but check the specifications carefully.

Amazon is a good place to start.

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Having a van with a battery, the only issue I had when fitting flat, penny round LEDs was that the fixing screws for the light fitting were too long and therefore fouled the new bulbs.
I had to resort to cutting the screw lengths down by about 6mm which still gave me plenty of "through".
R