Hi OP, my best suggestion for you would be Graco Slimfit R129 or Joie Every Stage R129. These are technically the same seat with different branding so just buy whichever is cheapest.
It doesn't meet all of your criteria but it is the best solution, IMO.
- Rear facing up to 105cm/21kg - this will last until age 4 if the children are not extremely tall.
- Converts to high back booster for children up to 145cm. Allowed to be used for one same child until the child outgrows (usually car seats should be replaced after 7-10 years' use, due to wear and tear).
- Good safety rating - most of the combination seats do NOT have this.
- Instructions are great and easy to follow
- Easy to convert between the different modes
- Seatbelt fitted, no isofix needed.
- Reasonable cost - around £150
Do check your RIB bed is suitable to transport passengers and fit child seats. I am not sure what the requirements are but I understand not all are.
Superfluous extra info/reasoning
Seats like the Cosatto are OK, as long as you use them totally to the instructions, which can be tricky because the instructions are often poor, plus the safety rating of the above Joie/Graco model is better. The main problem with the Cosatto with your requirements is that the seatbelt fitting is usually just for use in high back booster mode, and/or it gives you a lower rear facing limit, 13kg rather than 18kg (which is more like 2.5 years than 4 years), depending on the exact model. You can often download the manual before buying, so I would recommend this if you do consider a model like this, since a lot of them have similar names. Also, some of these seats need top tether attachment when used with isofix, which your campervan might not have - maybe check this. Cars manufactured since 2013 do all have it (and some cars since the late 1990s).
Seatbelt fitting for spinning seats usually disables the spin feature, particularly for rear facing mode. Spinning/isofix seats also tend to be heavy, which makes them difficult to move between cars even though isofix seats are easy to fit and remove, and it also means that the limits for rear facing are lower, usually around 18kg, which is more likely to be reached before 4 years old. Isofix support legs also must have a solid floor and if the distance to the floor is more than in a standard car, this can sometimes cause a problem.
In general, seats which combine too many features into the same seat and do it cheaply are a bad idea because there has to be a compromise somewhere, so if you're not compromising on the features or the price, then it's highly likely to be build quality/safety where you're making the loss. In particular, I would be extremely wary of what the German traffic organisation ADAC refer to as "white label" seats, where the seat model is produced by a generic manufacturer and sold under various brand names. Cosatto is one example of these, although their seats are mainly manufactured by Welldon, who aren't amazing but are OK. The major concern currently is a specific Chinese manufacturer who sells seats mainly on Amazon and other online retailers, the brands in the UK being Lettas/Reebaby/Reecle or simply listing the features of the seat without a brand name. Last year, ADAC (who provide crash test results to Which? among other EU consumer publications) crash tested one of these models with disastrous results and I've just seen an article, in German unfortunately, detailing that they have tested several more and found similar failings. The full report is due to be released at the end of May.
For this reason and also the advice to replace seats after 7-10 years of use, I'd usually advise against purchasing a seat with high back booster mode in mind for a 1-2 year old baby. You usually get much better value, safety and practicality by buying each stage separately. However at the moment, the models on the market are fairly limited, particularly when it comes to seatbelt fitting. If you did want to look into this, you might look at Graco Extend or Joie Steadi, which again are very similar technically. These are rear/forward facing seats up to 105cm/18kg which is about 3.5-4 years. Or Halfords have an extremely cheap and basic, rear facing only seat up to 105cm/18kg called Impala. The Graco/Joie seats are also a bit cheaper, lighter and more compact than the Slimfit or Every Stage, and will take you rear facing almost as long. The downside is that with all three of these, the rear facing mode has a shorter lifespan both in weight limit and also physical size it can be adjusted to. It might possibly be worth looking at 1x Slimfit/Every Stage and 2x the 18kg limit seats instead, then once the eldest or tallest child is approaching 105cm in height, you can reassess whether you need to look at high back boosters or extended rear facing or extended harness seats.
It might be worth considering whether any of the children are currently significantly bigger/taller than average for their age, and whether any of the families are considering having more children. If there are potentially more grandchildren expected, this is another argument towards seats specialised for each stage rather than all-stages type convertible seats. If you know for certain that at least one child will not last to age 4 with a 105cm limit as well (over 75th centile) then it might be worth already looking at one of the 125cm limit seats like Avionaut Sky or Britax Safeway M.
The cost is higher at about £200-270, but if you did for example:
3x Graco Slimfit at £150 = £450, or
1x Slimfit at £150
1x Britax Safeway at £200 (plus £15 spare tethers for other car)
1x Graco Extend at £80 = about £445
You would need to get at least one high back booster later on, but a basic booster seat equivalent to the one in the Slimfit is about £50, is lighter and less bulky, may benefit from ~4-5 years' future developments in car seat safety and won't have wear and tear from being used for 4+ years by toddlers.
Would definitely recommend consulting a brick and mortar retailer, either an independent (search stockists of Joie on their website) or John Lewis rather than Halford's aside from the Halfords-specific seat - the John Lewis staff training tends to be better. If you intend to fit three seats alongside each other, you will usually want to test this actually fits before buying.