The book I have been trying to remember was likely published in the 1930s. What i recall is that two children and their Mom or Dad were on vacation in the Caribbean. I think the title was Caribbean Caravan. The Caravan they traveled in was one of the old silver ones.
It might be a combination of two of Ruth Bryan Owen books:
In 1936 there is “Denmark Caravan” about family travels, illustrations show a silver caravan.
Later, in the 1940s, she wrote “Caribbean Caravel”, which seems to be about two boys going port-to-port, but details are scanty.