The size of vehicle makes a huge difference to how easy it is to park up eg at a supermarket or visitor attraction.
we tend to pick a site and stay a few days to a week before moving on , and tend to walk everywhere from the campsite but we do like a good walk. That makes 3 nights ideal I think.
We always have a few meals in stock for emergencies but like PP says - pick stuff up locally - it’s fun
campervan / motorhome selection ( assuming you hiring ) is all about endless compromise. And what works for one will be awful for another. We have a high bed which means we have a lot of storage under it for kit. Other people don’t like climbing to bed, so might choose a bigger van for the same walking kit , which then means less manoverable.
avoid taking too much stuff - pack like you have hand luggage only
ours has a fixed table for the back seats to use
most important - learn the basic rules. Don’t empty the loo or your waste water outdoors or in local toilets ( often a septic tank ) , don’t leave your waste behind, if parked in a general car park, don’t spread out - keep things inside your van. Driving on smaller roads , let locals past when you can