Is it the particle board itself being so heavy or the countertop affixed to the particle board?
Particle board itself is a bit on the heavy side. You could replace it with something like Mahogany, which would be lightweight and strong. As well, Mahogany is dimensionally stable, which means it won't warp. Other woods may warp if exposed to the elements (i.e. used outside).
While Honduras Mahogany (regarded as Genuine Mahogany) is quite expensive and an endangered species, Philippine Mahogany is much less expensive and more common (and you might know it by it's alternative name... Luan).
a 3/4" thick piece of Philippine Mahogany having your required dimensions would be less than 10lbs.
The only issue with Philippine Mahogany is it takes quite a few coats of varnish to fill the pores and finish, and it is a lot lighter in color than Genuine Mahogany.
Of course, there are a lot other options too.
The only thing that works on a RV is the owner...