So here's what I've decided on, due to the guy's who build them have taken them places I wouldn't even dream of...
Basically what this is, is an M923 platform, but with the original cab removed, and a MK48 "HEMTT" style cab mounted in front of the engine. This free's up more space to build my "rv" part, and also removes about 1 1/2 feet of height. This truck sits at only 8'8" tall, which isn't bad. Also, fun fact, since I will be turning this into an RV, I don't have to worry about a CDL, weigh stations, etc., since it will be titled here in AZ, and them's the rules here
I will be going with a 600hp ISX Cummins and a transmission and transfer case that can handle the power in the most extreme conditions. This also allows me to get very creative with my exhaust and intake. I will most likely remote relocate the radiator and use a hydraulic fan.
Since the payload of these trucks is around 20k, and the curbweight is roughly 22k, I can get frivolous with weight adding, which was a HUGE concern for me, since I plan on almost 500 gallons of onboard liquid storage. 200 for fuel, 200 for water and 100 for extra *back up motor oil, ATF, coolant, gear oil, etc* which will add in it's self 4200lbs.
With the ISX and a proper transmission I'll probably be able to net 6-7mpg maybe more if I actually drive properly, since it won't be tapping on the rev limiter to hit 60ish mph like the stock 8.3 Cummins.
I'm still in the planning phase, but I'm looking to purchase the rolling chassis, engine/trans, and cab by the end of the year, and get started on the fab work soon after. I will post a link to the build thread over on Steel Soldiers once I start for those interested. If I get enough interest I can put a mirror thread up in the "Projects" section of PSA.
I will keep track of every penny spent so anyone who's interested in doing a similar project will know exactly what they're in for money wise!