First off make sure to buy something big enough that you can live in it without killing each other. We have a 28 ft class A, in fact we are a week into a 2 week vacation in the coach as I type this, and while this size coach works well for us, after 3 weeks or so I think we both start getting a bit of cabin fever, and I doubt we would make it living in this limited amount of space for 6 month.
My second bit of advice is to have started shopping 6 months ago, but since you don't have a time machine, if you are going to do this, plan on buying sooner rather than later, this will give you time to shake out the bugs, to provision it for the trip, and to take a few shake down trips.
RV's are not like cars, don't expect everything to work when you buy it, regardless if it is new or used. There will be issues that you need to sort out, some of these will be relatively minor if you buy well in advance of your trip, or a real headache if you wait until the last minute and try to get them fixed on the road.
I don't know what it will be with your coach, maybe the side view mirror mount is loose and it shakes so much you can't see approaching trucks, maybe the furnace blower motor will short cycle or cut out when it gets warm, maybe the headlights will be stuck on low beam, or the cruise control will not work. Any one of these things could turn a pleasant trip into an unpleasant experience, and any one of these may be a simple fix if you knew about it in advance of the trip.