Robin it saves planty of time.
Last monday I did 2 full houses (stairs and 4 rooms), 2 x stairs and landing, stairs landing and one room, 5 houses altogether at same estate in 6 hours including break and emailing. Moving all the equipment, filing and emptying with buckets would make it 8 hrs work.
Plus its a marketing. Customer can see your great equipment nicely fitted on the van, thinks your machine is so powerfull if you use it from the van. Can not see negatives. I dont have many enquires for apartments and even if every second one still can be done from the van. If I need some small machine for apartments I will look after something lime santoemma maxi or bigger airflex mini.
The idea with preheating full tank not realy good, you will pay loads for the electricity. Cold water is a problem in winter as is too cold. Its enough to preheat just a bit, thats why I will use my heaters for winter only probably and only if job will require high heat. All using customers electricity.