Converting an aluminum enclosed trailer

Here is a forum for people who do just as you are wanting to do. Lots of interesting stuff here:
Tiny travel trailer forum

If you do it correctly, you can buy a good used 20X8 ft. for much less. And you will quickly learn that a 20 footer is a little cramped.

But, as one who has built and rebuilt many homes, boats, and other projects over the years, there is a certain amount of pride in saying "Yep. I built it myself"

a friend is modding a New 7x20 trailer V noise.. front is kitchen sink, mini freg, hot and cold water, tanks under unit. windows and a/c unit.

both sides have hinged beds.

for a 2 wheel and 3 wheel motorcycles in the center...

I have seen several closed trailer "upgrades" and some were really nice
one good thing is you can set it up exactly as you want. Bad news is if is doesn't come out great you cannot blame someone else. :)
If you decide to do it good luck on the project

It's certainly possible, and within the reach of reasonably adept do-it-yourselfers.

There are some campgrounds that may turn you away since it's not (in their eyes) a "real" RV. If you do a neat job of everything and the outside looks presentable, I rather doubt that would be a common problem. Many other campgrounds aren't so uptight.

There are some specific requirements for registering the trailer as an RV rather than a generic cargo trailer. In some cases, it may be advantageous to register it as an RV, as sometimes the licensing requirements are less stringent when pulling an RV than when pulling a horse trailer, and possibly insurance or registration expenses may be lower, and so forth. Generally, you need to have at least a few of a list of specific items installed: a permanent 120V wiring system, a permanent toilet with a holding tank (not a porta-potty unit), some cooking arrangements, some sleeping space, etc. Check with your state for their exact laws.

Along the same lines, some states are much more rigid about inspections and code enforcement and so forth for RVs than others. Some require certification as complying with the various building codes, and maybe inspections to the same, and others just take your word that it's an RV. Naturally, if you just register it as a plain old trailer, much of that doesn't matter one bit. You can haul a cot and a potty and a camp stove in a trailer most anywhere.

Neighbors did just that. They only camp at campgrounds and use their facilities. They insulated and put in a 15k A/C. They can take their motorcycle and golf cart. When they put in A/C they lost the skylight so they are thinking of putting in a couple of windows.

Just google horse trailer converted into camper there are plenty of interesting conversions out there.

You may hear from a lot of naysayers but there are many (hundreds) of folks using horse trailers as RV's and you are correct that they are sturdy, long lived and generally water tight. You will need electrical connections and I would suggest that you should consider a roof mounted A/C unit as well. Otherwise, you should be able to piece it together as you need.