Where in TT Premier can I deduct renovation improvement costs from the sale price of a mobile home (second home) to reduce capital gains?

Add the cost of the improvements to your cost basis. 

When asked for the basis of the home enter the Purchase price plus the cost of any improvements to the home prior to the sale.

The second home sale is entered in the Investment section of the TurboTax program.

To enter an investment sale -

Click on Federal Taxes (Personal using Home and Business)
Click on Wages and Income (Personal Income using Home and Business)
Click on I'll choose what I work on (if shown)
Under Investment Income
On Stocks, Cryptocurrency, Mutual Funds, Bonds, Other, click the start or update button