Window air conditioner systems and household split systems are far less costly than air conditioning systems that have been designed especially for RV use. If you are installing one of these units in a caravan on site, or an RV or mobile home that rarely gets moved, then you should probably check out the household versions that are currently being offered by other suppliers.
Here is an article that reviews the top portable air conditioners available today.
RV air conditioning units should only be considered by those who will be traveling. These are structured to keep water out when traveling and the fittings and mountings are designed for the continuous vibrations that traveling produces.
Rooftop models for caravans can fit into a 360 mm square roof opening or an opening that measures 400 mm. The 400 mm designs are usually made in Europe while the 360 mm options are designed for the 14″ American roof vent.
Although a newly built RV or caravan can be constructed to accommodate either option, the additional work that will be necessary to modify a current hole will be a major factor in your selection.
There is no need to make things more challenging than they already are. You should also consider how thick your ceiling is, but a number of options allow for a considerable range in thickness. This is something that will be included in the product specifications.