2nd Mortgage on Commercial Property

I'm looking for a short term 2nd Mortgage on Commercial Property to expand my RV Park. My property is worth $3m+. I owe $1.17m with a 30 year first mortgage at 3.5%. I need $500k to expand & open the RV Park on our amazing 45 acre property. 12 to 18 month construction loan would be plenty of time to build the pad sites, add the additional electric/water, rent the spots and secure financing (or equity partner) to take out the 2nd loan. Even a 20% loan would put my aggregate rate at 8.44% with the ability to take out the high interest piece quickly.

We're 12 miles from Downtown Tulsa. The RV Park just a mile south of us has 175 units fully occupied with a wait list.

Any ideas or directions would be appreciated.