If your mobile vehicle qualifies as a mobile home

the cost of the entire system can be deducted from your income taxes at the end of the year.  This can be a huge savings! As an example, if a fully installed system cost $10,000, you can enter that $10,000 cost into IRS form 5695 (line 1) which then allows you to subtract $3,000 (line 6b) from your tax bill for the year. That could be a big rebate and make your net cost just $7,000 for the system!

Even if you can't get the full $3,000 credit this year (say your tax bill was too low) the credit can be used in future years. Just be sure to complete for 5695 this year so you have a record of your expenses. 

And you don't have to live in the RV full time.

The IRS form 5695 defines a "Home" as where you lived during the tax year including mobile homes (which generally need just a toilet, place to sleep and place to cook to qualify.) 

Unlike the related "Energy efficient home improvement credit", which is also on the same 5695 form, the credit is not restricted to your Main home. 

(Your tax situation may vary - please confirm any of this information with a qualified tax professional - which I am not.)