Electronic – Switching regulator for 40~80V

Setting up parallel switching regulators is hard. They might not run in sync and start supplying each other. Also, you're saying you want 25A. You would need a ton of those regulators to make it work, if it would work at all.

My suggestion is to get some big coils and FETs that can handle currents like 30 - 40A and a buck-boost converter. Alternatively, you could begin with a buck converter and regulate to a like 10V, and then boost up to 13.8V again. Don't expect to find these regulators in a package that will supply, you'll have to add external components. Don't forget that a buck converter needs to handle 25*13.8/10 = 34.5A+ (not accounting effiency) , because the voltage is lower.

Here's a picture of an LT1425 flyback controller producing -9V. To get more voltage you increase turns on output winding of the transformer: -

This is a low power device that is probably suitable for your application but there may be others like LT3574 or LT3573: -

I know it's powered from 6V but it is producing 300V at 5mA and the positive output pin can be grounded. It has a minimum input supply voltage of 3V so should be capable of what you need with the appropriate turns on the secondary.

If you choose a design that has a flyback transformer it is inherently isolating input from output and you can choose to ground the positive output connection thus creating a negative output voltage.