I think you are going to be hard pushed for people to give you advice on this as the possible penalties for getting it wrong are very severe.
Have I operated a gas fridge (infact a gas freezer) inside a Designer? Yes. Would I recommend someone else does so? No.I won't
recommend it. it's your call.
It's up to you to decide.
Factors
I think are relevant.
You can now only use room sealed gas heaters within a caravan (God only knows what the regulations for a hybrid like a Dandy are) That is to stress a heater stoves still vent within the living area.
Before non sealed appliances were banned catalytic heaters and other small heaters could be used. These heaters could only be used within a room of a minimum volume or larger and required a minimum amount of ventilation.
Just done a search for Ubique Gas Heater but all the images I saw were of lapsed eBay sales so this image may disappear soon.
[You must be registered and logged in to see this image.]The heaters are about 800W
From
what I remember the room volume of a Designer would be sufficient but the volume of a Dart too small for one of these heaters. The information I got was actually off a different heater so it's all a bit iffy.
If you look at the figures a gas fridge consumes far less gas. Again I looked but it's a while ago. You check for yourself.
I often use a Tilley lamp to light the Dandy and to take the chill of the room. They are about 800w.
I have a CO alarm in the Dandy it has gone off three times.
1&2) In January 2012 when experiencing extreme gales to the point where the roof was lifting. The wind blowing through the Designer was such that if I'd got a gas fire in the middle of the floor I'd expect it to have sufficient venting. I was using the externally vented Riviera heater. Why the CO alarm went of ???????
3) I was using a large gas lantern glass about 7" in diameter in the Designer. The type meant to screw in a R904/R907 cylinder Quite a lesson.
The effects or more correctly the concentration can vary depending on proximity to the source. See the inquiry quoted below.
Frequent low level exposure to CO is not good either for more information about this and CO poisoning in general see
[You must be registered and logged in to see this link.] and
[You must be registered and logged in to see this link.] Things go strangely wrong look at this inquiry into a death in Scotland
[You must be registered and logged in to see this link.] The appliance was a room sealed fire and the inquiry couldn't replicate a situation that produced sufficient CO without sealing under the floor. I found it odd because there is a space under a parked car.
The idea is for the system to be safe and then use a CO alarm as an insurance policy. It is inherently wrong to run a system that might or might not be safe and rely on a CO alarm as your primary safety.
If using a gas fridge or freezer internally I'd suggest cleaning the flue checking the inlet air holes and inspecting the combustion to make sure you have a blue flame.
Hopefully you can see why I won't give you a straight yes or no answer.
Lighting Problems
I have a gas freezer (3 Way) that other than making sure the gas is purged upon changing cylinders is a cinch to light. I have a gas fridge that is a pig.
Make sure the fridge is level.
Purge the gas on changing or first coupling.
Check and clean the combustion tube and air inlets.
You saw the spark but for others check for a spark not just clicking.
The priming button. I found this on another website. On some fridges the priming buttons are just adequate but they wear in use so you might be able to smell gas but there isn't sufficient flow to light. Pack the inside of the button.
The gas is supplied through a regulator so a new cylinder shouldn't change pressure.
Substitute your regulator on another appliance and or a different regulator on the fridge. They do occasionally fail.
The fridge can be used under the bed. I'd suggest concealing it if you wan't the beer to still be in when you return. Puting a windbreak or skirt around the fridge??? If the skirt was tightly fitting and you were sleeping in the bed above see CO issues above.
The regulator has to be protected from rainfall.