Serious question for the seasoned motorhomers 👀

[Geek]

a pizza cutter

Pair of scissors works far better.

 

[Geek]

We reckon up to 3° is near enough, mostly it's for sleeping,

Thetford fridges claim to work up to 3° off level, which is really quite level.
We would be prefectly happy at 3° off level in any direction. It's not easy getting more level than that.
Dometic fridges are supposedly OK to 6° which is quite sloping.
At 6° off level, a coffee mug will slide across a formica table, and the van will be uncomfortable

however, Swift in their infinite wisdom placed the shower drain, and the kitchen sink drain in opposite place, what works for one doesn't work for the other to drain completely, level won't work either to fully drain.

Our shower has drains in opposite corners so ought to work at any angle. However it and the washbasin drained badly if the nearside was higher than the offside, until recently.

The grey waste tank had to be removed for some work on the chassis, and when it went back in, it just wasn't sitting right. So I took out all four (!) waste pipes leading into it, reseated the tank, tightened the brackets and refitted the pipes (including replacing a 90° elbow which must have been fixed with tape before I bought the van 16 years ago).
I didn't knowingly discover anything wrong, but it drains much better now.