The eTrek is a nice vehicle, but it uses a very proprietary combination of an alternator and inverters to run. Of course, there is the issue of running a Sprinter engine at an idle (need a high idle circuit so the DPF doesn't get clogged and throw codes.) Even then, I'd much rather put the wear and tear of constant use on a relatively (and relatively is the keyword here, as a diesel genset can be in the 8k range) inexpensive generator than on the Sprinter's engine.
There is one issue with diesel generators. On the Sportsmobile forum, a poster found an issue when the fuel went dry (under 1/4 tank) causing his diesel generator to run dry. He ended up having to bleed the injector until fuel was running, otherwise, he had RPM swings. Diesel engines are a lot less tolerant of running dry than gassers. So, I'd probably set a safety margin of 1/3 tank being time to flip the generator off and/or add more fuel.