First of all, nice conversion. I have a lwb vivaro. It's a little tight with a prochem tm in there but like you i sling the wands up under the roof. I don't carry a portable much now and you will find after getting used to the thicker hoses, you won't want to use one again unless you can't get in close enough.
Regarding driveways, as long as the fuel tank is full, it should not matter. Did a job last wednesday and was working out of the front door and getting very wet with the rain and just wanted to get home. Normally i would drop the waste water straight away, but left it. The next day i went to my first job, run the pipes then went to start up. Machine wouldn't start.
A quick apology then i rescheduled the work, took the van home and put on my driveway. I got out my tools ready to investigate, turned the key and it started straight away. I was relieved, so went to my next job. This was an old coffee stain which i said might not come out, and if not there would be no charge. I couldn't budge it, but the customer said they wanted the rest of the carpet cleaned as it was improved a lot.
Once again i run the pipes, turn the key and nothing. Once again an apology but no reschedule probably, so losy £75 for an hours work.
The problem was obviously the waste tank water. On the first job the road was cambered so the water in the tank would have tripped the cut out switch. My drive is flat so no problem. The next job was a steep drive so back came the problem.
I won't do that again.
Dave