Hello we will be living in a caravan once baby is 8 weeks old. There is no room for a bassinet or cot but the bunks have a mesh enclosure so we are thinking the baby will need to sleep in the bunk straight away. Can a baby this young sleeping any mattress or is there a particular firmness it needs to be? Thank you.

If there is no other option for a baby to sleep (cot or bassinet) then a firm mattress could be used.
How long will you be in the caravan for?
Is it possible you could use a portacot for sleep times?
There are a number of things you would need to consider when sleeping an 8 week old baby on a bed.

Red Nose recommends that a mattress for a baby needs to be firm, clean, well-fitting & flat (not elevated)
The firmness is important to ensure that baby’s head/face does not become covered by using a soft mattress.
https://rednose.org.au/article/what-is-a-safe-mattress

Ensure there is no risk of entrapment between the mattress & enclosure side.
How high is the bunk from the floor? Is there a risk of baby falling/rolling from the bed?

I have linked below which outlines general information about using a bed, however much of the information is related to a baby who is much older.
https://rednose.org.au/article/making-your-home-safe-before-moving-a-child-from-a-cot-to-a-bed

You may also find this interview about camping/travelling with baby helpful.
https://rednose.org.au/news/chief-midwife-jane-explains-how-to-sleep-baby-safely-whilst-camping