Normally, I'd say that 100K miles isn't all that bad, but it seems as if this truck is undersized for the FW, so it could probably be considered to have even more miles/wear on it than if it was a F-250 say.
Of course, if the truck and trailer were well maintained over the first half of their lives, and you also keep things well maintained, it isn't unreasonable for you to get the years that you want from it. There are lots of RVs with a lot more miles and years than that!
Look around a bit more, checking out similar units, to see if you are getting as good a deal as you can, but remember that sometimes the deal that is "too good to be true"...IS!!