Robin trust me it will be awkward as hell trying to leave the machine in the van!
How are you going to fill it? If you plan on bucketing water into it, it will be really tricky and awkward because you need to be above the machine to do so. That's almost impossible in a smaller van like scudo or combo, and even with more headroom of a transit you aren't going to be tall enough, you'll have to climb into the van. Then you'll spill some and get the floor all wet, so it will need waterproofing otherwise it will rot.
Filling with buckets also means you'll have to walk further back and forth to the machine.
But you could use a hosepipe, most TM setups have a hozelock self-retracting hose reel fitted to the inside. You could leave the loose end long enough to drop into your tank, and then just pull off enough hose to go to the customers outside tap.
But then you're putting stone cold water in the tank. In winter you're looking at nearly an hour to heat a tank full of mains-cold water, in summer still a good 40 mins or more.
You see how this is just making more hassle and not saving you any real world time at all?
Bottom line is 50ft max hose run on a twin vac, the booster is a crock of

because it boosts the wrong element of the system so it will do you no favours and will be yet another cable to plug in.
Sometimes it's 25ft to the front door from the van, and both jobs I did today I had 50ft to the front door. Not many houses I get to use 50ft straight from the side door to all the carpets. So for al that effort setting it up you'll still be dragging the machine off the van a lot. The only place whwre it would be a benefit would be terraced streets with narrow paths. But then you'd need a hose ramp, £300 worth of kit there so you see how this isn't adding up??!!!!
I used to run exactly the same machine as you and now run a TM so I'm pretty sure about what I'm saying

I actually used to use two ninjas with a Y junction on the vac hose so they ran in parallel. Was a stupendous difference over one machine, but a hassle to lug around and set up so only worth it on bigger jobs where it would save me at least 30 mins of extraction time. Awesome for suite cleaning though, compared to 1 machine.