We ordered and took delivery of a new 2017 motorhome in 2017, and specified solar planels. Is the cost of the solar panel option on our motorhome deductible?

Yes, Solar System for an RV and a boat have been accepted by the IRS as a second home for tax purposes. Click on Federal Tax Credit - Wholesale Solar

Qualifying equipment includes solar-powered units that generate electricity or heat water. The credit is only available for improvements you make to your residence. This can apply to a second residence as well. Costs for solar panels or related property that are installed as a roof or portion of a roof will not fail to qualify for the credit because they constitute a structural component of the roof (Instructions for Form 5695 - IRS.gov).

For additional information, click on Federal Tax Credit for Solar Energy - TurboTax Tax Tips & Videos

To enter your energy credit, click on How do I add or remove Form 5695?

Also, here’s the federal solar tax credit’s value over the next five years:

• 2018: 30%
• 2019: 30%
• 2020: 26%
• 2021: 22%
• 2022: 0% (10% for commercial)

Here are some examples of what you can claim:

• Solar Products
• Permitting Services and Fees
• Installation Costs
• Engineer Fees
• Electrician Fees
• Shipping Costs
• Solar Consulting Fees
• Tools Purchased or Rented
• General Equipment Costs (Fork-list, Safety Harnesses, etc.)
• Any Miscellaneous Costs and Fees (For Solar)

This guide covers it in-depth

"Is the cost of the solar panel option on our motorhome deductible?"

No, the cost is not deductible.  But as others have pointed out, you re probably eligible to take a solar tax credit.  A tax deduction and a tax credit are two different things.