Bongo Fury

I am wishing to install some lighting around the bottom of the sliding door area, so that when the sliding door is open the floor area by the step is illuminated and also would light up the awning tent when attached.

Found some good 12V led panels, but wondering where the wiring goes from the sliding door.

One option would be just to run another feed from leisure battery, down the passenger side and then have a switch by front passenger door pillar that just turns the lights on/off.
Would be nice to have it on the same circuit as the main cabin light, with same option of door/on/off, tapping into the wiring of the door.

Is that something that is do-able?

There is no wiring that will help you in the door itself.
The loom you want runs above the door and is IN2 it has a 12volt feed for the ancillaries i.e. the rear 12 volt socket, which may have been moved to the leisure battery of course, the Earth is Black and the live (fused by 15 Amp fuse top right) is Blue or the Blue/Red which is the lighting circuit fused at 10 Amps (top left).
I've used this feed for my awning lights with a switch positioned in the middle seat row (now unused) seat belt fixing hole.
The same loom IN2 also has the door switched earth on the Red/Green wire.
To gain access to the loom you need to remove the trim on the door pillar and running back over the nearside rear window, also you need to raise the inner roof, remove the trim running along the nearside that seals the roof trim to the roof - it just pulls off very simple - and then you can reach the loom itself by pulling the roof trim down a bit - be warned the loom is quite tight and there is very little spare to make use of - I used a pair of Scotchlocks (not my preferred connector I assure you) and as normal it took several attempts to get a good connection as the thin walled insulation is difficult to penetrate. You might want to consider breaking the wires and extending the loom itself using butt connectors as these are much slimmer and tuck back in more easily, you also then have much more wire to play with!
See this page and previous one;

If you can afford it (or your company can...) these are a great way to modify/extend car wiring.

The kit may be overkill, as it has sizes you'd probably never use (15mm - 7mm!)

Kit Contents (sizes in mm)
Part No. Quantity Stock No. Cable Size Max. Height Min.
B-155-3801 20 542-5599 1.5 0.76
B-155-3802 20 542-5612 1.95 1.1
B-155-03 20 542-5690 3.0 1.5
B-155- 05 20 542-5707 4.8 2.0
B-155-07 20 542-5713 7.3 3.3
B-155-11 10 542-5729 11.5 4.5
B-155-13 6 542-5735 15.1 7.0

Spacing didn't work there!

Links added to some of the useful sizes, in smaller quantities.