My Troll is fitted with a Truma Therme hot water heater https://www.truma.com/uk/en/products/tru...uma-therme.html This works well on hook-up and provides sufficient hot water for our needs. However, the manufacturer’s literature says that water will heat up via the passage of hot air through the blown air pipe that passes through the centre of the unit.....how effective is this in heating the water......I would have thought not very?

Not very indeed!
Maybe a slight warming effect but definitely not hot water.

I'm not sure why Truma bother with the hole through the middle , as Tony says it hardly has an affect. Surely it just ends up making the boiler larger than it needs to be for any given capacity. I would've thought that the priority for most things in a caravan is smallness.
At Mr Motters suggestion I had a Propex electric boiler fitted to my van, they are without trunking hole, in two capacities, have a digital thermometer plus the target water temperature can be altered. It's worked fine so far.

The Truma works fine on mains hookup and is located out of the way in the stern locker........it does annoy me somewhat that the heating pipe passing through may be more “ form” than “function”. I inherited the heater with the ‘van and, if it were my choice I would probably have gone with a different system....never mind!