Ive decided to play it safe and frame it out...was already planning on a partition
That's your best bet, normally the roofs have no real strength and you'll be lucky if it's bigger than 2x1 up there. I'm guessing there's a "skylight" of some sort so it should be easy to guess if there is one as you'll see the depth of things there and then you have the problem of them going the full width without support so will bend.
Or you could get a stud detector and then you'll be able to see the width of the studs. Bottom line is you would need to brace them before hanging anything over a couple of pounds in weight on them so you're just as well taking that further and making sure you have something to take the weight as well as having the space customised to the size you want with the ability to hang things where you want rather than be governed by the space between the roof studs.
Just be careful not to make your frame push too hard up against the roof, you don't want to split any panels on the outside due to pressure pushing up against the roof. And if it's an old battered wreck of a mobile home then it might be a good idea to go over all the external seams with some decent external grade sealant anyway as the old stuff, especially on the roof where it's exposed to sun and rain, could well suffer thanks to expansion from the obvious heat differential between inside and outside and you don't want moisture getting in there thanks to something as silly as old sealant.