My old lead acid coach battery is 10 years old and definitely on the way out… I’d like to swap in a 150AH LiFePO4 but sounds like adding a DC to DC charger is likely a must (my original alternator is rated just 70A, so I need to limit the charge, since the new battery can pull a lot of amps during charging, if not regulated). 
 

I like what I’m reading about the basic Victron 18A unit, but I’m not sure how/where I’ll mount it cause it’s not super small at 7.3” x 5.1” x 2.8” ! The Warrior’s battery box does not have room for sure. I’d like to just put this thing under the hood, near where the isolator/solenoid is now (which will no longer be needed), but the space available there is pretty small and irregular (not to mention all the engine heat). 

Anybody done a similar project recently and have a suggestion? (Or, are there LiFePO4 batteries that will self-limit their own charging current possibly? Back to Google…)

Not quite direct swap. But I good platform to start. There are several videos and photos as what you need to do to do that swap. I did it myself Two years ago. You will never regret it. I started with a dash swap before a motor swap. Direct plug and play. I love the extra gauges. I than added a Piller pod with tranny temperature and a outside temperature gauge. It took me about 3 months to do this project. I only used outside help with Tig wielding the exhaust headers. Modified the existing pipe. I also bought a wiring harness made.. So it was just plug and play. Started right up. There are lots of things you need off the 3.0 so don't throw anything away until your done. I used to 2002 3.4 but had to buy a 95 computer and throttle body. So you are ahead of me on that point. My total cost was $5,800. Than I sold $2000 worth of parts off the 4Runner that was wrecked. I also regeared to 4:88 gears.  I found a local gear shop and made a trade. Did the installation myself. I did drive it to a muffler shop and had a custom cat back exhaust made. That's part of the $5800. The most money was the wiring $1950 but well worth it to me. Good luck with your project.

There is a “frame and axle shop” near me that custom builds leaf packs. They typically work on fire trucks and large rigs but they did my alignment on a previous toyota camper. 

I think if you load your sunrader up with a weight you’d expect it to be when loaded up and then go have it weighed, you’ll get a front and rear axle weight. Take that number with you to a shop that builds leafs as well as your old leaf pack and they may be able to custom build you a pack.