This is really a matter of cruising style and preference. Some people feel that a Super Maramu's installed fuel capacity is not adequate for a world cruising boat, and others worry about fuel going bad before they use it. Neither approach is wrong, it really is a matter of taste. Some people fire up the engine anytime boat speed drops below 5 knots. Others are perfectly happy drifting along at 2 knots.
I know our boat has already been around the world--TWICE--with only the factory installed fuel tank. I decided not to add complexity--and weight--by changing an obviously successful design.
Don't forget, by dropping your speed from 6 knots to 5 knots you--almost--double your motoring range. Go to 4 knots and you--almost--double it again.
I don't have a recommendation on how to install a day tank, just an observation that it MIGHT not be the way you want to go. If you do want to go that way, I know several people on this forum have, and will have good ideas on how to do it.
Hollywood Florida, USA