My RV makes a good excuse for expanding the tool kit, but I've tried to not buy more socket sets until I actually need them.
Engine, chassis, cab fittings will be metric, whether U.S. or other origin, but they won't all be hex head. Expect some Allen head and Torx, maybe even the non-tamper versions of those. Most of the house hardware will be closer to fitting fractional sizes, though some will be metric. The actual hardware will likely not be SAE, rather the older standards used in household stuff. Mainly though, a couple of sizes of Phillips screwdriver, adjustable open end wrenches and a set of fractional hex keys, though you will only be using two or three of them. I find I am using my hex head nut drivers more often than sockets, and even then only a couple from the set go into my RV toolbox.