I don't want to trivialize the design task, but also realize this is a DIY question and so there is probably as much danger of overlooking the obvious by trying to address the problem as a commercial design problem (i.e. from the safety standards). If you are an engineer then have at it.
So in that context, I will point out some of the obvious and what is generally safe DIY practice. This may not achieve professional standards as I'm not familiar with them but you will certainly do much worse than these recommendations. I'm observing most of this in my own DIY build.
Rules of Thumb:
- Propane if flammable; can leak and needs to be outside or vented. The is a higher risk with high-pressure leaks.
- Lead-acid batteries give off hydrogen when charging; venting req the same as propane.
- Most electronics manufacturers warn of sparks and not to mount their equipment near flammable or explosive sources of energy.
- Note: Sparks can easily happen when connecting to a device that has capacitors (e.g. invertors and chargers). Depending upon a disconnect switch the may or may not expose a spark. These problems are aggravated by LiPO because of the higher rating devices that are typically used.
- If propane or lead-acid (even AGM) are inside the vehicle you should have them vented to the outside (preferably with a small brushless fan). This also helps with the isolation problem where you have a potential spark hazard.
- It is typical to put a propane canister/LA battery inside a closed box and vent to the outside.
- A high-pressure solenoid at the propane canister should help to reduce the risk of downstream leaks. Another option is to close the valve when not in use.
Minimize to the extent possible routing high-pressure propane around inside the vehicle. It is always preferred to drop the pressure down in larger distribution networks. One of the issues I have is with a MisterBuddy heater setup on a 20 lbs propane can; this is a high-pressure setup. I recently tried a regulator to adjust the pressure to the high pressure input. It worked.
This worked with a Little Buddy.
This type of device is advisable inside the vehicle regardless of the propane stowage method.
This is a solenoid that will control various gases and liquids at 145 psi pressure.