A 170hp 10 ton truck is obviously not going to have the same "guts" as 300hp 3 ton pick-up. If it means you can only drive at 70kph up a hill then so what. The advantages of something like a 1017 or a Unimog is in the gearing, off road ability and the strength of the chassis. My U1250 has the working gears, so low range first has a 52:1 ratio - multiply the 400Nm of torque by 52 = 20,800Nm - that's enough to pull a train - and that's what mine used to do. It is designed to be used at it's full capacity all the time, unlike the smaller FUSO/Canters, or normal 4x4's especially the compromise ones that are one the market now.
Getting back to the OP post - U1300 vs U100L - I guess it depends on whether you want something for an extended stay - months on end, or just for holiday trips. If you like camping and are prepared to live small and use the truck as a place to sleep in, then go for the lighter U100L.
If you want a camper big enough to live in semi-permanently, and must have all the luxuries, then you need a bigger truck like a U500 or a big MAN. If you want something in between - then a U1300L would be a good place to start.
Once you get used to a truck after moving from a car/4x4, then it seems to shrink. My U1250 seemed huge when I first got it, now I look at it and it seems small - especially compared to the U1700/38 and the U500's. I spent a few hour yesterday with a fellow Mogger who is building a U1750/38 DOKA - and that a big truck but with the DOKA, the camper will be the same size as mine.
I drove 1017's in the military. Empty. They are gutless. Just saying. But surprisingly good offroad!