Buck boost converter - High current 15A@12VDC +
While you may be able to get a 12V@15A buck-boost supply to work, the boost portion of that power supply is the portion of the design that will be very challenging.

Linear Tech does offer the LTC7812 which would supposedly allow you to do this (bear in mind the link is a press release!). You would need to cascade multiple units together to accomplish your goal, and that solution is designed to cascade outputs together. I have not personally used this part before so I can't vouch for the viability of that part.

Linear Tech does have a good reputation, and they probably have some development kits that you can obtain from them that would allow you to play around and see if it works. Once you are past that stage, it is likely to be fairly expensive to build this from the ground up; so unless you are building a lot of units you may wish to consider a solution that is composed of off-the-shelf components.

Have you considered using 24V automotive (truck) batteries and 24V-capable battery chargers? All of these components including the 24V->12V buck converters are readily available and can be sourced from retail outlets like Amazon.com.

I love the idea of rack-mounting this. Don't forget to add some cooling fans as dissipating ~30W of heat due to loss in there is something that you will need to think about. You'll want to ensure that you have some headroom in your battery capacity if you are gone for a long time, especially if the system is going to experience environmental temperature variations as that does sometimes affect battery capacity. This varies from battery-to-battery chemistry, so do your homework on that one before you build a bunch of units.

Good luck!