Yes I did gain weight on lithium. That is relatively common although each person responds differently to each medication. Weight gain can occur on Seroquel as with all atypical antipsychotics. Sleep apnea can occur for a variety of reasons but weight gain can be associated with it. Perhaps your psychiatrist could adjust or change your medications as needed. You could discuss that with them and see what might be of help.
All atypical antipsychotics can cause weight gain and so can I think some mood stabilizers. The atypical AP that I took that didn't cause weight gain was Geodon but this is different for everyone. I've even started eating less and less and I'm still gaining weight on Abilify.
It sounds like some mild exercise would help. Just walking for 30 minutes a day broken into two - 15 minute walks will help. Remeron will help with the sleep problem. Good luck!
I know Seroquel has a side effect of increasing your chance of diabetes, so it is possible it may be contributing to weight gain. I would say weight gain or not it is a good idea to exercise a bit if you can. Not only will it help your pancreas it will also help your mood. I know it's hard sometimes.
It is so ironic we take these meds to feel better yet then they make us gain weight. :(