You haven’t stated what type of horizontal tank is being considered: DOT portable type or ASME RV/motor fuel type.

The portable type has to be removed to be refilled.   This could become a real pain if mounted on the roof.  … The ASME RV/motor fuel type can be permanently mounted and refilled in-place.

Nor, have you indicated if the tanks are for a liquid or vapor system.    … i assume you are looking at a vapor system.    I see no reason vapor  tank could not be mounted on the roof.   The line pressure is a regulated vapor pressure, therefore, height above the equipment would make little difference.   … Just remember it has to be refilled.

I suggest mounting the horizontal tank in a compartment with an access door to the outside.