You don't need to explicitly use samtools for CollectRNASeqMetrics from Picard (http://picard.sourceforge.net/command-line-overview.shtml#CollectRnaSeqMetrics)
the command line should be following (after downloading and unpacking Picard tools):
java -Xmx4G -jar picard/CollectRnaSeqMetrics.jar REF_FLAT=exome.refFlat INPUT=aligned.bam CHART_OUTPUT=rnaSeqCoverage.pdf OUTPUT=rnaSeqMetrics.txt
the exome.refFlat file for H. Sapiens could be downloaded from here.
for other species check http://hgdownload.cse.ucsc.edu/downloads.html
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Please check previous questions: Extracting Read Count For Each Gene/Exon From Rna-Seq Bam Files