Property Tax Contesting/Appeal Companies

I know there are quite a few companies out there that will contest your property taxes for you and take a fee if they win.  Does anyone have recommendations for a company they have used in the past and had success?  

I did an informal hearing for a mobile home community I own in Iowa where I got the assessed value to go down $20,000.  The only problem is...I was asking for ~$500,000 reduction in valuation.