Create a Constituent Query with this criteria
Prospect>Proposals>Campaign (or Fund) equals XYZ

NOTE: This is a query for a group of Constituents who have a Proposal for a specific Campaign or Fund.  If a record has multiple proposals, they may appear multiple times in the results due to the many to one relationship.  A query is a tool to provide a group of records, a Constituent query will return results of a group of Constituents, not proposals.