As said some of it will always be there. Clean all hard surfaces with a mixture of bleach and water. Clean all wood surfaces with Murphys Oil Soap. Replace all cushions and the bed mattress. No one will want to consider sleeping on a mattress or sitting on cushions saturated with tobacco smoke. This even turns off some smokers.
The carpet? Cleaning will help, but you'll never get all the smoke smell out of it. I've looked at cars that were owned by heavy smokers, where the dealer tried to shampoo the daylights out of it. The mixture of cleaner and tobacco odors just made us sick. No way would I buy something like that.
Another good product for the shower doors is Kaboom.
Are they replacing it with a new(er) trailer? Suck up the low value on a trade. Too much work on this one to make it worthwhile.