I do like the idea of a boost button so as standard it is float charged but user can select boost when time to charge is important. It would be rather easy to do charge current could hold on the boost latch and auto drop out as set current.
I have questioned the current normally set at 10% so with a 25A charger it will drop to float once charge current drops to 2.5A however in real terms it's the battery not the charger which should set the level so with a 110AH battery around 3A but with 40AH battery 1A I think the chargers should have dip switches to set charger to suit battery.
The priority system is all well and good where all three can provide the same power but if you have solar panels as a way to extend time when on a site with no huck-up then where there is huck-up then that would be priority one. Solar panels are heavy and the idea of having a roof full of solar panels which would then increase fuel consumption while towing does not really make sense it's a caravan not a narrow boat.
Towing power is also questionable with a car using standard split charging the solar panels may produce more than the car by time volt drop is considered. OK with a 12v to 12 v inverter like used to charge bow thruster batteries it will work far better than solar panels but how many people use them.
The priority would need to be dip switch controlled as set with the solar panel and car split charging type taken into consideration.
However back to the main problem who would decide priorities? I am sure that with a float charger both car and charger can supply the same battery together only when a stage charger is used is there a problem where back feed to the car could cause a problem with the engine management detecting over charging and taking what it thinks is appropriate action.
The real problem is the solar panels as to date I have not found any solar panel controller with simple float charge so as soon as the car is connected the solar panel must be switched off. So question is if the solar panel is integral with the caravan is there any point in charging from the car? Far safer to just have no connection to car so can't upset car engine management.
Clearly with a proper 12v to 12v inverter there can be no back feed to the car so you could use bow thruster type inverter but unless one is a real heavy user of 12 volt power this does seem a bit of over kill.
I also saw it as an interesting challenge and was surprised there were no reasonable priced commercial units. Talking to a fellow radio ham and he tells me he has a supplier of 3A anything in (zero to 24 volt) to 12 volt out switch mode units which are quite cheap but 3A is really a bit on small size. Next time I talk to him I must ask about using multi units as this would work OK with 6A that would be plenty. Three hours at 6A = 18AH and that should be enough to work water pump for a good few days and likely more than standard split charging would pump in.
Years ago I had a fleet of Library vehicles each one towing a trailer with batteries in trailer for lights. They had split charging but although on plug in we would measure a healthy 10 amp charge within 15 minutes this was down to 3 amp at that time 1970's there were no switch mode units to charge batteries and all attempts failed to put in enough charge. The only method that worked was to take a three phase tapping from the alternator and take it to two blocking diodes (one input unused) and from there to trailer and moving the tapping on RB440 CAV regulator to Hi. OK with this started at 30A (from 60A alternator) and dropped to 10 amp after 15 minutes which worked of sorts. However this was a steep learning curve and I realised how poor most split charging systems were. With the first boats I worked on also had problems the cables from the blocking diodes to batteries had to be exactly same length or it would burn out blocking diodes. Latter boat used two alternators one for each battery and still latter used the special Sterling inverter and today narrow boats can be charging aux batteries at 120A on first starting engine. But our caravans are still split charged today in the same way as they were in the 1970's in the main so in real terms very little charge gets into the battery.
To get a good charge from towing vehicle some thing like this is required the plugs complying with BS AU 149a and BS AU 177a or BS EN ISO 11446 are really not man enough for 50A charge and although used on wagons I don't want a connector like that on my car thank you so with an integral solar panel on caravan I would not even try to charge from car.