Welcome to the forum, quatto

I am not a techie

To the best of my knowledge, 3 way fridges (12V, gas, 240V), can not be regassed, as they are a sealed unit
The sealed unit can be replaced

Most caravan repair places, have access to a local gas fitter, who will be able to check/service/repair your 3 way fridge

Hope that this info is useful to you

I had some problems with my 3-way fridge and while searching for answers on the internet I came across the correspondence below on a UK forum. I cannot vouch for the accuracy of it because my problem turned out to be something different, but they seem to know what they are talking about, and the upshot is that 3-way fridges can't be regassed...

There seems to be some confusion as to whether it is possible to replace lost refrigerant in an absoption type unit. Refrigeration was my field years ago and I hope to clarify the situation.
It is not possible to regas an absorption type refrigerator they do not contain gas as do compressor type refrigerators,which contain a single gas, and regassing is straight forward. Absorption units contain a mixture of 3 components to make them work, water, ammonia and hydrogen in very precise quantities, and an anti corrosion additive usually zinc chromate. The problem is that it is virtually impossible to replace the components in the exact quantities, and the manufacturers of these units do not give the required information to make this possible coupled with the very expensive equipment neede to carry out this work, the cost would be prohibitive. The construction of the unit is of a completely sealed steel unit, there are no seals or valves to leak. the pressure on the condenser side of the evaporator "U" tube is in the region on 200psi so if a small leak does occur then the mixture of gasses will be lost very quickly.
When working with refrigeration plants I sometimes used to work with large commercial absoption refrigerators and we sometimes had to turn them upside down after a period of non use this would help remove any vapor lock in the system and after a day or so they would usually start working again,but this was quite a few years ago. The main reason for a modern absorpsion type refridgerator to stop working if the heating source is ok is that there is corrosion in the absorber, or generator or there has been a loss of pressure, if the unit has stopped working whilst in service with the heat source working then it is usually due to an irrepairable fault in the absorber unit. If the unit has been lying unused for a period and does not work then turning it upside down for at least 12hrs and then trying again it may start working and continue so for a while but the performance may not be as good as it should be so a replacement will be on the cards. These types of refrigerators give many years of excellent service, and the newer models are a great improvement on the old ones.

I have spoken to our local Dometic service engineer who stated that they cannot be re-gassed, he also asked if there are any signs of yellowing at the back of the fridge ie this would denote coolant leakage. there isnt any i can see.
The mixture of refrigerant can over time of inactivity crystalise and block the tubes some are quite small this is probably the cause as the fridge has ben standing some time.
He did state that inverting the fridge may sort the problem as the crystels will return to their liquid state but it should be for a number of days unfortunately my time scale does not allow for that.
My other option is to replace the whole of the refrigeration unit containing the refrigerant this would mean a complete rebuild the cost of the part would be around £260 this is an option i will explore when i return. 

-- Edited by Mamil on Thursday 14th of February 2019 11:00:34 PM

Had a 3 way for nearly 40 years and occasionally failed due to gunk build up. Strange, that. I thought the 240v would still work it, but not a skerrick of cool. A simple clean out of the jets by a plumber solved the problem both times. Due for another shot now, actually.