Is it legal for a bank to repo a trailer if customer is not behind on payments but has paid payments late each month?

One would need to know the terms of the agreement with the bank to properly advise you. I can think of one scenario right off. If your agreement states any late payment incurs a late fee of X. Let's pretend your payment is 100 a month and there is a 25 late fee. You pay 100 every single month, but are late every single month. end of payment month one, 100+25 is owed but you paid 100. You start month 2 at 25 in the hole. end of payment month two, 25+100+25 is owed but you paid 100. You start month 3 at 50 in the hole. end of payment month three, 50+100+25 is owed but you paid 100. You start month 4 at 75 in the hole. end of payment month four, 75+100+25 is owed but you paid 100. You start month 5 at 100 in the hole. end of payment month five, 100+100+25 is owed but you paid 100. You start month 6 at 125 in the hole. If this is your situation, because the note deficiency keeps growing, eventually someone will call the matter.