I was just looking at it quickly and giving a hint, no "case-sensitiv-copy&paste ready" code.
I personally spawn MT targets without xml, just fine.
Code:
local MainTankTarget = self:SpawnHeader("oUF_MainTankTarget", nil, 'raid, party',
'showRaid', true,
"groupFilter", "MAINTANK",
'yOffset', -16,
'oUF-initialConfigFunction', ([[
self:SetWidth(%d)
self:SetHeight(%d)
self:SetAttribute('unitsuffix', 'target')
]]):format(140, 18))
MainTankTarget:SetPoint("TOPRIGHT", oUF_MainTank, "TOPLEFT", -20, 0)