Hi Toolara and welcome. We have a 12 foot with extra annex etc and I agree van parks are a battle. This pic is at Mission Beach and you can see the conc slab poking out from under the annex.
That is the whole setup up, we had to pre-book a large site, but it was a family do and we had 3 sites in a row.
We have been on smaller sites but you can only use what you can get up, and it's a pain with small sites and concrete as I don't want the main area on the slab in case it rubs the floor, (we do have shade cloth under it though).
Basically what I'm getting at is what Jules said, be prepared to haul the trailer round by hand a bit to fit, ask for the larger sites (no cheek no xmas pressies), or pick sites, (like we did at the bottom pics), where you can gain a few inches on a corner, or maybe look at a trailer that can be used with or without the extra rooms occasionally.
Plenty of good reasonable trailer mobs is Brissy, Trackabout, Customline etc who can add rooms either out the back or drawbar sides, or is it a Shak that sleeps them like 4 separate rooms. The other options for the growing independants is maybe a good tent for them which could sit under or partly under the annex.
I have to agree a 12 footer is a dilemma in finding floor area though. The bottom pics are at the Cairns Coconut van park.
On the electrical side, new, some have a standard, but most will provide, if possible, what you ask $ for.
Not trying to tell you what to do, but sometimes I get an idea and forget to look at alternatives so hope it helps, and space vs accessories like kitchens and storage etc is another thing.. sometimes you've just got to try them.
Paul