Leisure battery...

It's difficult to remotely diagnose, especially with a van that old, because the documentation was not very good, typically.

Do any of the other 12V appliances work? Do the lights work when you connect to a 240V hookup? (apologies, a '88 van may not have 240V hookup but I don't know for sure about yours).

Thanks.

the main hook up works fine - the internal lights only work off the leisure battery though I think.

Are they all switched individually or all at once from just one switch?
If just one switch, get behind it and measure there, if power at switch, then if power at its output when on. Could use a jumper lead to power +ve from vehicle to lights, thus bypassing switch, it lights then work its the switch???

Each light has its own switch and none of them work.  There's no power reaching any of the lights....

Well, you've done the fuses, so, unless there are others (entirely possible!).......

I'd take a wire from LB 12v+ to one of the lights, ideally at its switch, and see if that makes that one work.

Also whilst multimeter is to hand, do you have zero ohms between LB -ve and vehicle chassis?

Also does the LB voltage increase when EHU is switched on?

It's going to be a process like this I'm afraid, but my gut feeling is that there's a common power point for them which is not getting power?

Just had another thought....
Our Executive has a single switch by the cocktail cabinet end of the lh bed.
This isolates all LB loads. Used mostly to put out the last light/tv at night
We had no idea we even had one till we found and tried it!!!
Could that be the problem????