Yes, there are a few small trailer parks in Belize, mainly in Corozal and Cayo. But don't expect U.S.-style or even Mexican-style parks.
More often hotels and lodges will rent space for you to park your RV for awhile.
I can send you a list of campgrounds if you like.
A few people do bring their RVs down as temporary living quarters, in some cases while they are building their houses. Of course you will need permmission if you park on private land, and you will need to take some precautions against theft.
The Northern, Western, Hummingbird and part of the Southern Highway are paved and fine for RVs. Most other roads are not paved and can be rough on a rig.
Belize First Magazine http://www.belizefirst.com/