What I've found so far is that William Deuchars & May/Marjory Johnman had at least 6 kids: Robert (my ggg grandfather)b. 27/9/1811, c. 6/10/1811 Dunning, who married Agnes Horn, William b. 10/10/1807, c. 15/10/1807 Dunning, James b. 10/6/1814 c. 21/7/1814, Alexander b.17/1/1809 c.22/10/1809 Dunning, Peter (your gg grandfather) b. 17/1/1820 c. 23/1/1820 and John b.29/11/1821 c.9/12/1821. (Info from familysearch.org)
Can't find a marriage between William Deuchars and May Johnman, but births of children listed as lawful so am assuming they married.
Have found an Alexander Deuchar who married an Anna Mercer/Moxey in 1767 in Forteviot who had at least 4 children: William b.24/12/1780 Dunning, Anna b. 5/6/1785 Dunning, Janet b.17/8/1770 Dunning and Alexander b.2/9/1778 c.6/9/1778 Dunning. No mention of a Robert. Don't know if I've gone down wrong track there!
I have a William Johnman and Margaret Marshall being parents to Marjory/May Johnman. Interestingly their other children were Christian b. 14/8/1776 Dunning, Elizabeth b.12/3/1764 c.18/3/1764 Dunning, Anna b.22/12/1778 c.27/12/1778 Dunning (?was this your Ann who married Robert), Margaret b.25/2/1768 Dunning, James (?Peter's uncle) b.6/3/1781 c.11/3/1781 Dunning, Robert b.18/12/1770 Dunning, William b.21/9/1765 Dunning and Alexander b.18/12/1773 Dunning