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.