Lots of thoughts on this. There are a number of good posts about larger or aux gas tanks. Some nice installs.
What I did was buy a good rear bumper with swingout for the heavy ass spare tire. Provision for 2 Jerry Cans (10 gals). Antenna and jack mount. Could do both and haul water in the Jerry Cans but you'd need to beef up the rear suspension and then of course gotta do the front suspension.
Concerning the 'heavy ass spare tire', all that suspension work be a waste without bigger tires/wheels. There is no end.
More gas is good. That long trail you've been following has a log blocking it and you gotta go back.
Yes we found a way around. Did I mention buy a chain saw? But we didn't use it on the big log.
I'm now amazed at the number of aux tank options that exist - both in the US and especially some of the beasty tanks from Australia. Tonnes of info on here that's going to give you hours of pleasure, but just to give my "executive summary"
- I've had an aux tank on now for 12 years, including some pretty heavy offroad use followed by about 8 years non-use with gas still in it (this last part is not recommended)
Plus: It's transformative for distance, especially if you also carry a jerry can or two - you just never have to worry (in the US lower 48) - it made my long range trips much, much lower stress
Negative: always slightly worried that I might trip up on emissions testing, but it hasn't so far; some people report ODBII codes showing up with them, but I never have; it's a moderately invasive mod
Definitely try and get one with an external fuel pump - my early gen MAF tank is internal and it caused some concern when it looked like it had died after my vehicle had been non-operational after 8 years, but everything came back to life after a couple of cycles.
I think most aux tank systems work the same way as the MAF in that you have a dash gauge for it and you have a dash-based transfer pump switch that drives an electric transfer motor/pump - very easy to operate. I think some have separate fill necks, but the MAF has a replacement filler neck that can fill both tanks from the stock location (my MAF does cause most gas pumps to start clicking off long before the aux is filled, so some pump-it-and-see is needed until the gauge reads "Full" on both tanks).
I had my local Toyota dealer install the Man a Fre tank this spring and i absolutely love it, no more having to find a station before i head into the hills. For me it is just piece of mind that i can head on logging roads or whatever and not need to worry about Jerry cans, etc. Have had no issues with mine so far. I did install a new suspension when i got it as i put a new front bumper on as well.