The idea isn't totally silly. I don't think its practical, but it is doable. First, you need to determine how many BTU's the refrigeration system needs. 5000 seems to be too much. Pet-cools run 2000BTU's, and I've heard 2nd(3rd?) hand that someone is running one in a teardrop in Meixco. So... 1500-2000BTU/hr seems to be a good goal.
The little LP powered fridges you see only put out around 250btu/hr. Thats why they take so long to cool down. You can find a propane unit that has the capacity, such as this freezer
. 1700BTU's, but it costs $1700. These units also have several drawbacks. They are large, and they only run when perfectly level. These units also use 3 liquid/gases. (Ammonia, water, hydrogen) Any leaks, and they stop working. For $1800 (or $2500 for the DC breaze) you have a 12v off the shelf unit.
Your idea about disconecting the refrigerant line is not going to work. The best you can do is to have a water/glycol heat transfer system. The AC system sits outside, and the water line runs to the heat exchanger inside the trailer. This water system can be used with any of the AC units, if done right. You could also expand it to use it as a heating system.
I've been thinking about this a lot recently, and I've come up with seveal solutions. Some easy, some almost imposible for an individual. With some hard work and injinuity, you can also get a propane powered A/C system.
Easiest is to get a wall AC unit and duct it in. You can find examples all over the forums. The Pet-Cool is a sweet system, with the only drawback of using AC power, and more expensive than a wall unit. Good thing about it is that it will not cycle on/off as much.
Ammonia/water absorption system. (Lithium bromide/water as well) I'm not talking the gravity system the refrigerators use, but the pump driven type that industry uses. Most of you cooling power comes from the propane heater, so electrical power useage can be as small as 20-35 watts. DC power drives 2 fans and a pump. This system can be as large as you design it to be. A/C, refrigerator, FREEZER all in one unit. Its not a project for the timid. As soon as I get my trailer done, this is what I'll be working on.
Duplex Stirling cycle refrigeration. Stirling cycle engines are called external combustion engines. Heat one end, cool the other, and they work. Starting site for information is
These engines also work inreverse. Apply mechanical movement, and you get cooling. The duplex system ties two of these back to back, so you use heat to get cooling. They're not as efficient as a compressor, but then again... you get propane powered A/C. You can add a small generator to the system so you have power for water pump and fans so this unit is outside of the trailer. Hmmm... throw the hot end in the campfire?
There is another cycle, like the duplex stirling, but I'm suffering a brain fart. Something like ve.... it doesn't have a piston, just two displacers. After a starting nudge (or small drive motor) this thing will move heat from the cold end to the ambiant temperature heat exchanger.
An article on direct thermally driven heat pumps is at
A more advanced heat pump is the free piston duplex stirling cooler. It doesn't have any connecting rods between the displacers and piston. Heat one end, cool the middle, and the other end gets cold. Add alternator coils and a magnet to the piston, and you also have electrical generation. How about a generator that's as loud as a furnace/water heater?
As I said in the begingin, its not totally silly to think about propane powered A/C, but you're not going to get it with a few modifications to a small refrigerator.