I remember some Class A motorhomes that had a dual line master cylinder and two small hydrovac units. Front and rear brake systems were independent, but they required a pretty complex plumbing system. It would probably be more effective to keep the original system up to "as new" condition, and fit the driveshaft with a big manual disc brake setup. I don't remember the name of the company, but there is an aftermarket driveline brake that has enough power to lock up the rear wheels on a loaded truck.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!