No matter which lifestyle you choose, you must register in Spain after 90 days in the country. Living under the radar becomes a nightmare, the moment anything goes wrong.
If at retirement age registering means you get access to health care and other benefits. If choosing to live in a park that offers the same services as Park La Posada, you can relax because they deal with all the paperwork for you.
If living in a resort or town-village, then the services of a “Gestor” will be useful and each town has several to choose from. You can also use them to change your UK driving license from the UK one to a Spanish one as that will also need to be done if planning to retire to Spain.
The days of living in Spain full time but pretending to be a tourist in Spain are long gone. Older UK registered cars are stopped routinely now, so it is always best to buy a Spanish registered car to use with the steering wheel in the right place, of course.
For motorhome owners, you will not be able to register your UK RHD motorhome onto Spanish plates because the home door opens out into the road. You can register (import) a UK car though, and it is up to you if the expense in doing so is worth it.