Ankush,
Looks like you are trying to shove a collection of custom objects into RoamingSettings, which is not quite what it is meant for. Local/Roaming Settings are stored usually in Registry and meant for simple name-value pairs.
How about this - you take your entire object model and flatten it out for storage as a File? This way, you can easily serialize/deserialize your data and hydrate/dehydrate your object model in your App when needed. Also, the flattened content can be saved in the Roaming Folder for syncing across multiple user devices. Simply annotate your custom object properties as needed and use the DataContractSerializer to flatten/unflatten your data and persist an XML file in Roaming Folder. Just do not depend on an instant cloud sync; Windows will sync the file in Roaming Folder opportunistically.
This MSDN quickstart should help:
http://msdn.microsoft.com/en-us/library/windows/apps/xaml/hh700362.aspx
Thanks!