We do have a couple of auto body members here who might be able to provide some insight on how to locate and clean the drain holes.
@NYBODYMAN
@mrbill55
I think there are others but can’t remember without see some of their posts.
According to Auto-sleeper the black plastic bit is bonded into place when the fibreglass cabover was fitted to its not removable.
The drains are just slots in the black channel here:

They correspond with the roof channels on the panel van roof.
looks very nice inside! Congratulations and happy travels!
Its very dated, well it is 30 years old, so will need a bit of modernising. Doing something to the cupboard doors will probably make the biggest difference.
I think just painting the beading round the doors and replacing the sticky back plastic inserts with something else and fitting new catches and knobs will make a huge difference.

Upgrade to LED lights and new sockets and switches too.
I found this van for sale thats had the whole interior redone so i might steal a few ideas from it.
https://www.quirkycampers.com/uk/for-sale/coco-beautiful-boho-beach-hut-on-wheels/
Looks very nice, inside and out. Good layout for travel, and for camping. I like it! Hopefully your knowledge of VWs will transfer.
Its pretty tidy overall. Definately on par with some that were twice the price.
Theres a few bits of trim (the one for the flue on the water heater mostly) that haven't survived too well so will need replacing. Flue cover is obsolete so need to find a good seconhand one somewhere.
I'm quite tempted to raise it a little (maybe an inch or so) and fit some A/T tyres on it. Definately gonna upgrade it from solid discs and drums to vented fronts and discs at the rear as its 2.8 tons in weight fully loaded.
Maybe get rid of the lower side stripe decals and go for a forest or mountain scene thats so loved with vanlifers to make it a bit more distinctive.
Need to have a proper look at the clocks. Its in P but the display shows D and doesn't change. Hopefully an easy fix.
