Without seeing what kind of space you have (photos/dimensions helpful), kinda hard to make a recommendation. Home Depot and Lowe's have a pretty extensive stock of various sizes of vanities and sinks, but with an RV, my thinking is that it will have to be custom built, which as the previous poster said is costly or a stocked one modified. Nothing seems to be a 'standard' stick house size that will work.
Take a look here at surplus and salvage dealers : RV Salvage
Some one of those folks could help
Dave W along with my DW, Susan and our poodlepups, Callie & Molly,2011 Ford F250 6.7 CCLB, 5er Hitch Option w/B&W Hitch,,Ride Rite air bags, 2014 Montana High Country 343RL (38')