If my boyfriend gave me a trailer with the bill of sale signing it over to me can he legally take it back ?

If the trailer was an unconditional gift, he has no right to take it back.  However, he may claim that the facts are different.  For example, he may claim that the trailer was a gift in contemplation of marriage, which in some states (I don't know about Alabama), assuming that the Court believes him and not you, might give him the right to get the trailer back.  The bill of sale is strong evidence on you side that the gift was unconditional, and may (again, depending on the state) preclude him from testifying to anything contradiction what is written on it, but if not there is no guaranty that the Court will believe you.