Haven't done that mod. But our last coach had the tank mounted vertically. Can't think that would be a problem and that it would function better as water is heavy and air bladder would push better.
I haven't seen a pressurize tank in any of the newer coaches. Because of that, hot water heater PT relief valve weeps/leak. My Suburban water heater manual says if PT valve dripping to install a pressure tank to control expansion. Good luck
Ret. Military/Corporate Pilot
Key West Snowbirds-AB 450HP
"Don't cry because it's over, smile because it happened”