Mr B
From memory, the Transit Connect LWB is almost the same loadspace footprint as the standard Scudo/Dispatch/Expert.
As Richie says, watch the weights. The general concensus seems to be that you would be wise to aim for about 1.2 tonnes capacity.
I've been researching vans recently and found that the entry level Transit, VW, Vito, iLoad along with most others weigh in with less than one tonne. The Scudo and other versions are available with a suitable weight capacity. The best footprint, weight capacity and value I've seen so far is the Ducato/Boxer/Relay at almost 1.2 tonnes.
I've seen a Transit Connect with a Hydramaster 318 and they had to remove the passenger seat. I've seen Dave Ingram's brilliantly laid out older type Scudo with a Woodbridge TM and it was superb. Also seen the new Phoenix in a Scudo and it has a smaller footprint than my Scorpion porty with auxilliary tanks.
I think that the Prowler has the potential to make a superb T/M in a medium sized van, but I've only seen them in larger vans. It has a small footprint and most medium vans should be able to accomodate an on-board water tank. If you're man enough, you could also wheel the Prowler closeer to the door if parking access is restricted.
On the weight front, even if your chosen van has sufficient capacity, you'll need to look at weight distribution over each axle. I had a Phoenix in the back of my Dispatch just behind the bulkhead and even without water the distribution looked too much biased towards the front axle.
Safe and happy cleaning

The Ken