Needing advice

This is a long one boys and girls

Owner Operator for 2yrs now with 10plus years of professional driving. We run PO, I have a situation I could use some advice on. Last year 50miles down the road pulling a old step deck the rear quarter caught fire. No real damage was done to the cargo but the axles, tires, air lines, bags and electrical were damage on the rear drivers side. Now insurance is refusing to pay saying I didn't have the correct coverage and now the bill has gone to a collection agency who has contacted my company demanding payment.

No if it was something do to the negligence of the driver I could understand. 50miles down a flat road the driver notice smoke from the rear trailer wheel well. Pulled over just before the fire started and did what he could till the fire department arrived. Now there was no hills, had full are pressure and did his pre trip check before making it 50miles.

If the driver was at fault for negligence something would have happened from the start of the trip. If brakes didn't release or air lines where hooked wrong there would have been a signe. 50miles down the road sounds like faulty equipment to me or equipment failure.

Early 70's trailer and what they want to repair it you could by a new trailer

Tell me what you think and open to suggestions.
P.S. happened in Cali