Anyone remember the member who bought the pretty giant host and had a 250 and took long trip to pick it up? He got a lot of good advice (similar to the above) but was already committed to it and rolled with it. The reason I mention him is that he was very open and knew what he was getting into, and he followed up and filled us in on how it went and provided a pretty good perspective on it both beforehand and afterward. It would be a good thread for reference to the OP.
Does a new Lance 1191 really only weigh 3500? I think my tag shows something like 4300 and it really weighs more than that. I can't remember exactly what it weighs since I care more about GVW but it is more than 4300.
As far as I know it is not possible to re tag a truck once the tag is put on. It can only be done before that tag goes on to begin with. I didn't do extensive research but it quickly became clear to me that it was a dead end. My 4500 is rated at 17500 and the 5500 is 19500 with the only difference being the rear springs and I really wanted to make the spring change and get it tagged because I run about 19k. Can't happen. No big deal.
Anyway, just make sure to weigh it all like Bka suggest, the numbers don't lie and having that data is what you need to check against the tire capacity which is your first order of business for safety sake. I expect the single best thing you could do after driving it away as a unit would be to think about 19,5 tires.
You will probably love the camper. We sure do. Compared to BKA we hardly use our camper but we do have something like 500 nights in ours. Good luck and let us know how it goes please.