Re: 12V vs 110V
I would try and get an LP alarm that runs on dry cells like 9 volts. I just like the idea of them being totally independent of all other systems from a safety standpoint. It also makes it much easier to eliminate parasitic battery draws. When you store the bus you could just pull the battery out of it rather than have to disconnect wires.
As for what is more efficient for everything else...well...no option is really more efficient. If the fridge draws 250 watts at 120 volts it will draw 250 watts at 12 volts for the most part. What's important is to design around your camping style. I think you're going to find that most pumps are 12 volts and I think that would be fine. They are low enough draw that I wouldn't even worry about it if you didn't have house batteries.
So...that leaves only the fridge to worry about. That's where it really comes down to where you are staying and such. If you plan to mostly camp where you can tie into shore power or you're going to haul a generator around then I think a 120 volt fridge will be the easiest, cheapest solution. The downside is that you will see those inverter losses when running off your battery bank going down the road as was posted above. But you won't need a big battery bank so long as you're staying close to shore power.
On the other hand, if you're going to be boondocking a lot I would suggest a 12 volt or propane fridge (or any combination of the 3 power sources). The reason for that is that you will have a large battery bank if you're boondocking. You might also have solar or wind generation, both of which operate at 12 volts. Basically, you would be eliminating that 10-15% inverter loss which could mean the difference between needing an extra solar cell or not. The downside will be when you park near shore power. If you don't have a 120 volt option you're going to be stuck relying on your battery bank, propane, alternative energy, or using an expensive converter to go from 120 volts to 12 volts.
I run my fridge strictly on 120 volts. Typically I'm not too far from shore power. I also have a generator I can use, albeit very loud. If I'm boondocking I can go for 2 days on my battery bank. I can recharge it to ~90% with the bus alternator if I choose or I can recharge it with my tractornator (as well as supply real time power) which is a surprisingly quiet and fuel efficient option.
The point is that you really need to decide for yourself what is going to work best I guess.