Cabins and RVs should have the maximum solar capacity feasible. This ensures batteries will charge fast and deeply even with intermittent sun. The battery capacity required for cabins and RVs needs to cope for about three days. From thereon it is cheaper to charge via a grid-voltage charger driven by a small inverter/generator. See: RV Solar and Alternator Charging.
As a rough guide, (and assuming a 12-volt system) do not allow battery capacity (in amp hours) to exceed 50% of your solar capacity (in watts). Unless you do, your batteries may not likely be adequately charges. A 100 amp hour battery thus needs at least 200 watts solar capacity. Seemingly excess solar capacity will assist charging during times of little sun. There is no risk of overcharging, the solar regulator prevents that.
Dual solar system
AGM batteries are ideal for cabins and converted coaches, but their 33 kg per100 amp hour (for 12-volt units) is too heavy for small caravans. Where size and weight are critical use lithium-ion LiFePO4 batteries. They cost more, but as 80% of their nominal capacity is available, that cancels out their higher price. LiFePO4 batteries are also light and compact. They are about one-third the size and weight of AGMs. Another benefit is that unlike conventional lead-acid batteries, AGMs and LiFePO4s can be left for up to a year without use.
If towing a camper trailer or caravan consider a self-contained solar system in each. This enables you to have one remain in the sun. The other can be in shade. You can readily interconnect the systems to share power and battery capacity. This also enables you to have a small fridge in the tow vehicle. It is very convenient when shopping. This also works well if using a vehicle to access the cabin. That vehicle too can have a solar array on a roof rack and battery capacity too.
Our 4.2-litre Nissan Patrol and TVan (shown above) were set up this way. Each had its own system. They could be used separately or paralleled if desired, but (in over seven years) we never needed to do so.
It is also feasible to have alternator charging. : outlines how.