We've run our onboard genny no more than 5 hours in a year of fulltiming. And most of that was 'on exercise' to keep it functioning.
We have a 4kW built in Onan, using an entire large locker. If I could find somewhere secure to leave it in UK, I would remove it next time we're back.
This would free up a lot of extra locker room, plus allow up to 100kg of payload to go into the new locker.
In practice, I'd probably buy a 900W Kipor type suitcase genny to replace it for any battery recharging required when wildcamping. The advantage of this is that the baby genset can be run remote from the van, behind a hedge or wherever, to minimize noise; and a little genny sips fuel rather than gulping like the Onan.
I think the main reason RV's have such large gennys as standard is to run the rooftop aircons. Without this requirement, the loads are limited to a kettle or hairdryer (plus battery charging), so a much smaller genny + invertor will suffice.