I think cost is the main problem, inverter generators are good, so are solar panels, but do you need them? If I can I use sites with hook-up, however I have been caravanning since 1956, and would spend whole of school summer holidays in the caravan with my mother, dad visiting some week ends.
We had just one battery which had to last 7 weeks, yes we could use rubber bung in floor to pump water if battery was flat, and we did have gas lights, why they ever got rid of gas lights I don't know. However we rarely used gas lights or foot water pump, it was just for emergency use.
Today no longer foot operated water pumps or gas lights, and even gas fridge and central heating still needs a 12 volt supply to work, however unless you try using 230 volt stuff with an inverter off caravan battery one can do a week without recharging, and most sites without hook up do offer a charging service.
So I have been looking at how to use non hook-up sites, I would want a hook-up for one night before returning home as need the motor mover, although could get a battery to leave at home and change battery.
But when you remove battery for charging you want some thing to replace it while it is recharged. Could be the tow car, but also could use a battery pack, I already have a 20 Ah battery pack so all I need is a S type 7 pin socket to a cig lighter plug and this will provide power while caravan battery is being charged. Cost under £5 using what I already have.
At £1000 for an inverter generator used 3 weeks in the year with hook up costing less than £5 a day that's around £100 per year, so 10 years before it brakes even not counting fuel costs, so more like 20 years with fuel cost, I can last 3 days without 230 volt power, I got damp in electrics some where so could not use hook-up. So using sites without hook-up enroute is not a problem, if you really want to use it without hook-up, then cheaper to buy old caravan which still has gas lights and foot pump, and do it up.