I believe all crowns are 12v.
I did quite a lot of research before I bought my Crown. I was comparing 4106's when I was shopping. I went with the crown so I could tow 10000# trailer since there's no frame on the GMC buses. I valued the toughness and unique nature of the crown, and managed to sell my wife on it. I also plan to boondock a fair amount once I'm retired from the military.
4104 pros:
More storage space. Bays are these mythical things I fantasize about as I fight for every cubic inch of space in my interior.
Smoother ride. Although I don't have any complaints about my crown's ride, I've heard the Jimmy's are much smoother, and I'm afraid to go ride in one and find out what I'm missing.
More room. 4104 has straighter walls - easier to convert, and you end up with more useable interior space.
More experience. Lots of people have converted these or owned them, they are well known, and there are lots of floor plans to choose from. Lots of good info when you have questions or problems.
Cool factor. Old transit buses are neat, and they have a certain aura to them. That old apple pie america thing.
4104 cons:
V drive and all that other rear end drivetrain weirdness that I don't fully understand. I'm also not a fan of detroits only because no one knows how to work on them anymore.
No real frame - not an issue if you're flat towing a prius, but lots of stories of cracked buses from towing or not bracing them up properly for a roof raise.
You need to go to a bus/rv place for work due to the unique nature of the parts/layout. Which can result in raping of the checking account quickly.
Crown pros:
Sports car of buses! I can take 55mph corners at 70, if the wife isn't with me.
Bigger brakes. I think brakes are more important than power when it comes to towing. Crowns have class 8 truck brakes. Rumour is the 40 footers can lock them up on the highway, although I don't know why you'd want to. My bus will practically stop in the length of itself.
Maneuverable. My 35 footer will turn tighter than my dually pickup. The 40 footers, not so much though.
A real frame. Although you have to fab the rear part, you have something to build off of.
Uses standard truck parts. You can swap in modern truck transmissions with overdrive, more gears, whatever. Same with axle ratios. You can make the bus as fast or as torquey as you want. You can get brake parts and everything else at any truck repair place, no need to find bus specialty spots.
Aerodynamic. Every crown guy I've talked to gets better mileage than what I have heard about in the Jimmy's. I get 10 even running 68-70 in the hills using the floorboard cruise control.
Cummins engine. The 855 cummins is the chevy 350 of HDT engines. They were used in trucks, tractors, combines, cranes, generators, everything. Also the govt used about a gazillion in various vehicles as well. So parts are everywhere and cheap as it gets, although that won't last forever, as fewer people are working these engines. But normal people can still work on these - you don't have to cross your fingers that you'll only break down within a 100 mile radius of Don Fairchild.
Crown cons:
Storage. I don't even know what that word even MEANS.
Skoolie stigma. Some rv parks don't like skoolies, and even though your bus wasn't one, it's something you have to overcome. Resale value suffers as well.
Short people only. I think a roof raise is pretty much mandatory for anyone over 5'7".
Gulmite fasteners. ARRGGGH!
Auto trans. I don't know that much about the autos. I think that one would be a MT644. I'm not sure how much towing capacity it has. However, more people can drive an auto, so there is that.
Sorry for the long post. But you asked. More people on here can shed more light on the GMC's than I probably can.
