If you need a 24v to 12 v converter, I highly recommend the Wilmore brand. US made, great quality. Used by fire departments, airports, railroads, emergency vehicles, etc. I use their models 1640 and 1416 (versions up to 35 Amps or higher).
Running 12 v radios and equipment directly off the Wilmore converters works slick and is much less hassle that keeping a separate 12 v battery charged (the way I did it at first).
For an inverter (to 115 vac) consider a used UPS (uninterruptable power supply). Lots of these show up after the batteries go bad. Find a 24 volt one. Feed the S250/truck's 24v replacing the batteries (verify the charging circuit can handle the heavier batteries) and don't plug it in to get inverter 115 out. Back home, just plug it in and get 'shore' power, topping up the 24 v batteries and maintaining them.
My application is in 24v Unimogs.
Tennmogger
Need some advice on inverters. I have a 24v apu off of the M577 command post track. My plan is to use it on my S250 commo shelter, with an inverter and a voltage reducer mounted inside the shelter. This arrangement would give me 24v for the mil radios,12v for some ham radios and 120v for other radios as well as lights. My main concern is an inverter which provedes clean power. The apu is rated at 4.3kw