It would be better if you had the receipt, a cancelled check, or a bank statement showing the expense added to the basis of the second home. You can go ahead and include it as part of the cost basis. It is unlikely that you would be expected to justify the amounts that you enter as the cost basis for the second home.
Just keep in mind that if your return were being audited for any reason, everything being reported is subject to further scrutiny. If you did not have the documentation, then the numbers would be adjusted at that time.
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