Norway is one of the most expensive countries in the world and visiting it is not a budget anyone can do.
Best way to visit, but not for a long time is by living in a tent and buying fast food unless you have access to a kitchen or buying finished food in grocery stores. Resturants is way to expensive.
As for laundry while living in a tent, the best way i can think of is to rent 1 or 2 days airdnb room with mashing machine or go to the mostly only self laundry service i know about in Oslo, but still that cost alot.
Another way is to wash your cloth in a bucket with soap or if you are in the mountain and having access to a river, you can maybe wash there.
Living cheap in norway is not a good standard. Airdnb prices goes from 550 euro a month in total for a decent place, by most europeans thats also alot of money.
Traveling by train and bus is a good option, bus is cheaper and if you stay in oslo the subway is good to get around fast.
You can get so much more value and the same in Sweden, so is it really worth it?
Unless working in Norway, traveling here would bite in your budget and would not be comfortable on a tight budget.
This is my best thoughts on how to live cheap in Norway, maybe there is more ways to live cheap, but as a norweagian i never found it.