If you are only looking to have SP to maintain a battery charge in storage, you should be able to get by with a smaller set up.
But, as much of the costs associated to a good SP install are labor, adding bit extra power to provide the option for more times 'off of the grids' (Not just for boon docking type usage. But I also looked at it as sort of 'insurance' for disaster type situations when power may not be readily available for a period of several days.), maybe worth considering.
AM Solar, yeah other side of the country, is a good source for info to both read, and to also talk with about putting together a kit for you. They send it, and they could probably also help you source a local installer to do a competent job too.
Even if your coach was prewired, so much depends on what you end up installing, it may not be appropriate. And, it's a very minor costs wise in the process.
One other opinion. Future proofing with a bit better sized cables, and or solar controller - will allow you, or a potential future buyer, to add more solar panels if major boon docking is ever needed.
Knowing you from other postings, you do things the right way - and that is proper in this mod too
(Just a tip, be sure to mention in your needs/designs, that you want to keep both the House and Chassis batteries charged. Many times, only the 'house' battery is charged. Remember, your 'trickle charger' is part of your 'plugged in' charging system. It is usually (Maybe, but not always.) gets the House up to snuff first, then diverts charge power to the chassis battery. Make sure the 'chassis battery charge' during storage is known as a key need too...)