As there is no invoice number, could you not just use the Trailer VIN as the reference number, as it is unique to your transaction with Escape.
2017 Escape 5.0 TA (someday soon, I hope
2015 Ford F150 Lariat 3.5L EcoBoost
2009 Escape 19 (previous)
“Most folks are about as happy as they make up their minds to be.” — Abraham Lincoln