I stopped in Luxembourg for an overnight on the way to Interlaken, it didn’t go to plan though as the campsite closed early but we found a nearby 24 hour carpark used by vans.
Nightmare to drive around Luxembourg city as Satnav can’t deal with stacked roundabouts (and neither can I).
Very cheap fuel and that’s about it as far as I could see, plenty of people filling Adblue containers etc with Diesel.
In Switzerland food and drink are very expensive compared with France, check what you are allowed to import into Switzerland as there were quantity restrictions on meat and dairy, nobody checked ours, just as well as we were loaded up.
You can buy the Swiss Vignette as you enter Switzerland;
Motorway vignette Switzerland - prices 2021 - Tolls.eu
Here’s a good campsite just outside Interlaken with excellent public transport connection around Switzerland;
Camping-Ferien Wang Beatenberg Switzerland Käthi und Ruedi Knecht, bus travel pass included in the campsite fee.
There is a small tourist tax on all campsite fees.
Wild camping is only allowed above the tree line, if you can get a van up there you might want to get a job as a tank driver.
Travel passes including Jungfrau are expensive, these are easily bought at Tourist Information so you don’t need to buy before you have a weather forecast.
I think a busy 6 days is about right time to see the area around Jungfrau.
In 2020 there were more Chinese tourists than anybody else, they don’t queue willingly, but they do understand ‘FO and get to the back’.
We left Switzerland and went into Italy and further.