I think you have something belongs to meI think后面you have.应该是宾语从句 在宾语从句里面you have something that belongs to me that引导的是什么?状语从句还是定语从句?那是否可以是宾语的补足语？I fully believe that he will do what he has promised这句是否和上面一样呢？that后面是宾语从句？do谓语，what he has promised是修饰do?应该是状语从句？

you have something that belongs to me
that定语从句
属于我的东西
fully believe that he will do what he has promised
that后面是宾语从句
do 是宾语从句的谓语，what he has promised是do的宾语，是宾语从句

定语从句

that引导的是定语从句,从句缺主语,也就是这句话在意义上来说,something就是从句的主语.原句可以拆分成：you have something.Something belongs to me.
I fully believe that he will do what he has promised
这句话that引导的是宾语从句.后面的what引导的也是宾语从句,这是有特殊疑问词引导的宾语从句.也就是说,do这个动词的动作对象是（what he has promised）“他所承诺的事”.
这两句话里边都没有你说的状语从句,看来你对状语从句的理解有误.状语从句
的类型很多,有时间、地点、原因、目的、结果、条件、让步等类型,总之所以的状语从句在整个句子中都是充当状语的.你说的两个句子里都没有状语,更别说状语从句了.