"VINs were first used in 1954.
From 1954 to 1981, there was no accepted standard for these numbers, so different manufacturers used different formats. In 1981, the National Highway Traffic Safety Administration
of the United States standardized the format.
It required all over-the-road-vehicles sold to contain a 17-character VIN, which does not include the letters I (i), O (o), or Q (q) (to avoid confusion with numerals 1 and 0)."
13 digit VIN's were common prior to 1981, so the number you found could be correct. I'd check with Holiday Rambler. If all else fails, many states will provide you with a new VIN. Check with your BMV.