No way to do it through your setup and have it go through your breaker box and the rest of it. Your wiring configuration/capacity isn't set up for it. However, I've got a friend who wanted 50 amp like I have so he can run dual AC.
What he did was take everything in the galley, all outlets, microwaves, coffee pots etc and pulled them from the breaker box. He then installed a standard 110v plug next to the main 30amp plug. So now he has an outlet next to the main line. He built himself and extension cord that has 2 male fittings. One at each end. He plugs the extension cord into the 20amp side of the outside box and into the new outlet. The galley is now off the breaker box inside and checked at the box outside. This leaves him with 30amp for everything else. Lighting outlets stuff like that. DO NOT put the water heater on electric. Let it burn propane. It only uses a small amount of propane and once hot it rarely kicks in anyway unless it's pretty cool outside and/or your using hot water causing it to run more frequently but even then it's still not all that much. Also using CFL for lighting is a huge gain power wise. Using only the 12V DC lighting is probably even cheaper powerwise. He then wired for a second AC. It's worked out rather well for him. Running both AC units eats up much of the 30amp but there's enough room left for lights, television and various other things like satellite TV or stereo etc. Poor man's 50 amp.
