1. Math camp at the University I attended used Simon and Blume - Mathematics for Economists. We had to turn in a problem set (of around 100 problems) over Chapters 2-9 on day 1 of math camp. Then in math camp itself, we covered chapters 10, 11, 12, 13, 14.1-14.8, 15.1, 15.3, 15.4, 16, 18, 19, 20, 23, and 30. I did not use all that much of this material again. I had only had Calc 1 and Calc 2 prior to the PhD program, so math camp was a struggle. My cohort mate had only had business calc, so it was an even bigger struggle for him.
2. Ask senior PhD students what the expectations are. At my University, the PhD students in the years ahead of me told us that getting a C or two in the PhD econ courses was not uncommon. As far as I know, none of the faculty in the accounting department knew what grades we got in the econ courses, besides maybe the PhD program director, but I do not even think he looked. But this will vary from school to school. I can say that my cohort mate, who only had business calc entering the PhD program, got Cs in a couple of the PhD econ courses. I did not get any Cs. He has 7 A publications and went up for tenure early. I have A pubs (nothing close to 7) and am certainly not going up for tenure early. So at least in this anecdotal case, performance in the economics coursework had little correlation with our eventual publication success.
You are going to be time constrained. When faced with a trade-off of whether to put a few extra hours into an econ PhD course problem set or into your discipline's doctoral seminars, put the extra time into your seminars. I believe you will get more return on the extra time spent familiarizing yourself with the literature of your discipline vs. spinning your wheels for a few more hours on a problem set.