Hi Christine,
Making an initial 6-month visit to check things out is a good idea. You'll have to make the trek into the London Consulate for your interviews for the visa anyhow - and you can ask about a multi-entry visa when you're there. They used to be fairly common (for business people) but it's hard to find much about multi-entry visas on the State Dept. website lately.

If the Consulate folks can't advise you, there are any number of immigration attorneys in and around London who might be able to advise you on the best type of visa and what to carry with you to "prove" your good intentions.

Also, one caution on mobile homes (I lived in one - in Indiana - for several years) - they do attract tornadoes and hurricanes. Not literally, but in the event of severe weather (which they do have in Florida now and then), trailers tend to take the brunt of the damage. (Locating inland, away from the coasts is probably a pretty good idea.)

There are condos in Florida where you can hire the management company to handle the various bills while you are back home. Or ask any of the real estate agents you talk to in looking at property. It costs a bit extra, but in your situation would probably be well worth it to have someone handling the various fees while you're away. There may also be the option of renting out the property when you're not using it, to cover some of the costs. (Hey, you won't find out if you don't ask!)
Cheers,
Bev