Comfortable sleeping arrangements

Hi,

In 6 weeks, I will be embarking upon an adventure to Mornington in the Kimberlys. I'll be up there to do field work on birds, and will be camping at the one site for 2 months. I currently have a Kathmandu self-inflating mattress, which is fine for a week, but I find any longer than this and I'm getting stiff and sore. I'm tossing up whether to try a thick one of a better brand, or go for a 4wd mattress. Considering my 2 months will be a whole lot more enjoyable with a decent night's sleep every night, and that I won't be moving around at all, what would people recommend?

Thanks.