Asterix
I'd be tempted not to write off a 3.5t straight away.
When I chose my Movano (and others will have similar) I opted for the bigger engine. Nothing special at 120bhp, just not the bog standard 90-100bhp. It's still good for 95ish on the autobahns and no issues at all with the various steep twisty alpine roads in Germany and Austria. Plus, with that type of driving, I still averaged 38mpg around Europe.
What length is your hab area? I'm looking for 18 feet,give or take a foot or so,most of the 3.5 tonners seem to be substantially shorter,and the extra length I'm looking for would probably wipe out any load capacity.