I have a 6.5kw LP generator in my MH and I don't have any issues with it. The tank is 35 gal if memory serves and I've never run out. If I had a choice I would probably go with diesel just for the sake of convenience. I would check into a diesel conversion if this one ever goes belly up, but that would be the only reason to change.
I usually buy propane at one of the big local propane joints. I can't really speak to price but I believe the last time I filled up it was way less than diesel or gas. I have never tracked hours per gallon, maybe somebody else will have figures on that. I have propane on board anyway for cooking and the fridge so I still have to buy it.
Our MH has a great floor plan and that is the reason we bought it. I guess what I'm getting at is I wouldn't pass up the right MH just because it has an LP generator.
If you do end up with an LP generator do yourself a favor and swap the fuel line from the liquid port to the vapor port if it hasn't already been done. Less hassle with fuel line clogging etc.