I had a 4.8m ally centre console that I built from a kit. I had a 50 litre under floor tank connected to a 50 hp Yamaha 2 stroke.  

For the first few years I had no end of problems with moisture getting into the tank. Especially if I had not used it for a couple of weeks.  I ended up just running 2 portable 25 l tanks and no more problems. Very rarely had to connect up the second tank. 

If I was going any distance I just loaded a plastic Jerry can.

 Thanks for the input mate.

Was discussing the issue with the missus.  Her response was "what about the spare 25 litre tank in the shed? You can use that for short trips and just in case, bring along the 20 litre jerry can.  For long trips, you can fill up the big tank!".

Sometimes I think the wives know more about boats than we do! Haha.

So there you go, case closed!  Gonna crawl back under a rock now! Lol

 My boat a Whitley Savage Centurion 4.8m only came with two portable 25l tanks, and I wanted more so weighed  up whether to install a under floor tank or not. I ended up just buying two more 25l tanks for two reasons. Cheaper and also to be able to rotate tanks and keep the fuel fresh. Also can distribute the weight where I want. Most trips I only use  two unless I go out to the shelf but it's good to have exta when it comes up rough.

in the tank, you have to keep it filled up, as an empty tank has air and therefore moisture.
You will have issues with water keeping it empty and only filling it up occasionally as your wife has suggested.
You could just try and keep the inbuilt and use a fuel stabiliser, I personally don't know how they work.

 Deck space.  if you want to carry a lot of fuel, it takes up some deck space, inbuilt definitely tidier, and some boats simply don't have anywhere to put it. Older style glass boats usually have space under the well for it out of the way. I only carry two on the Dommi, as you'd be going a very long way on an F115 to need more than 54 litres in a day. I have more tanks, use the rest of the space for a kill bin instead.  I have seen large tanks installed in this space, but that must surely be giving the owners issues with weight distribution, they  like it forward, and 120 litres right down the stern can't be good.

Tote tanks may not be seen as cool, but I know I'll never have fuel contamination/tank leak/fuel sender issues creep up on me good alternative for sub 6 metre boats with the right deck layout.

 Some very good points mentioned, especially with weight distribution as needed! 

The most fuel I've ever used was about 80 litres for a full day bouncing around Direction bank, annoying the Sambos! Haha. That was with a 150hp 2 stroke. Current motor is 120 hp 2 stroke so i doubt i will use that much fuel again.  Most of my fishing is river/bay/inshore.

Will change over to portable tanks.  Anyone interested in a 120 litre Ali tank in very good condition, pm me! Haha

 EDIT:  The existing setup has a fuel filter/water seperater. Im assuming I should continue to use it? Or will it be best to connect the portable tanks directly to the outboard?

 have enough hose on the portable tank hose to plug it into your tanks, then attach to filter separator unit. Best of both worlds, otherwise you are purely relying on the motor's little strainer unit for filtration.

 Thanks mate, much appreciated.