By the way, what is going to change next month that he will have the money but not this month?
See Isaac's reply above. We've made a several large purchases over the years (since retirement) for which we had plenty of money available (not immediately), but that required us to liquidate some piece of an investment in order to actually be able to spend it, since we don't keep huge (car sized) amounts of cash on hand, or even in a simple savings or checking account. So depending on the investment, the time of year, and other factors (including tax consequences) it can take from a few days to two or three weeks to get the money from that investment so that we can use it.
That being said, having to wait for funds means you must be very careful about scams (well, you should be careful anyway, but...).
RV's are more than a couple hundred dollars so even if the story is sort of legit you are dealing with someone who doesn't have their you know what together!
I don't pull those investments out until I actually find what I'm looking for (weeks, months of search time that the investment can be earning more), and until I actually know how much I'll need. So it can, indeed, be the case, unless I'm going to finance the purchase, in which case, even with a pre-arranged loan, it'll take a bit to complete the deal, especially since I'll often use certain investments as collateral in order to get lower interest rates.
Of course I'd want to put down a deposit to hold whatever it is for a certain period, but that's something I'd have to negotiate with the seller.
In other words, the fact of having to wait for a bit to get the funds out isn't, in and of itself, reason to say there's something wrong, it's just reason to be extra careful, IF the seller is willing to wait.