Though several conceptual inventories have been developed for thermal physics, none target upper-division material and all focus specifically on thermodynamics without including statistical mechanics content. In this paper, we outline the development process of an upper-division thermal physics assessment that captures both thermodynamics and statistical mechanics content. To ensure that the assessment can be broadly usable, as a first step in this process, we administered a survey to physics faculty to determine the scope and content variability of thermal physics courses across institutions. We received over 70 responses from 63 unique institutions, approximately half of which are minority-serving institutions and women's colleges. Our findings support the claim that there is significant variation in the content covered at different institutions, but also some general agreement on a number of core content areas. We identified 10 key topics which were listed by the majority (95%) of survey respondents to focus on for the assessment development process. We then wrote assessment objectives that encompass core content goals within these 10 topics, which guided the writing of free-response assessment items that were piloted with students. Using student responses to the free-response items, we developed multiple-response versions, which includes both multiple-choice and coupled, multiple-response items. In this paper, we present details of the faculty survey, including methods of developing and distributing the survey to solicit a broad range of perspectives. Additionally, we present results of the survey, including core content covered by faculty in upper-division thermal physics courses, and discuss how these results were used to guide development of assessment objectives and assessment items. We include the full development process of one assessment item as an example.