Rookie 'Vista Cruiser' Owner / LP Generator Questions

I'm looking at buying an '07 Vista Cruiser Mini and have a couple of questions hopefully someone can help me out with:

-With an LP Generator (I've rented dozens of times and all had gas gennies), off of the 12 gallon tank that the RV has, how long would this typically last if the A/C was used for 3-4 hours during the day? I know, maybe a ridiculous questions to try and answer, but versus a gas generator that I recently used on a weekend trip where it was run for 30 hours and used maybe 1/8th of the tank, am I looking at running out of propane quickly with this? Are these not meant to be out off the grid for 10-12 days? I'll be in the Nevada desert very far from any services at least once a year for this 10-12 days... I'm trying to make sure that I'm buying something that is going to be able to handle that. Maybe the answer is to bring a supplemental propane tank that I can rent from Airgas for this one trip each year?

-Is the Sprinter Chassis (Mercedes Engine) known to be reliable or are there lots of complaints? I can't find many on the forums that I'm looking on and I don't know any RV owners that have this type of RV - all my friends have Ford/Chevy engines. Anybody out there with any advice as to whether you'd stay away from these? A conversation I just had with a friend was what if I had a problem while out in the middle of Nevada - how many shops would be able to work on this Mercedes Diesel engine...? I didn't have an answer for a good question.

-Any advice whatsoever about this particular model helps... I'm looking at an '07 with 40k miles. It's in great condition; I am also spending good money on it for what I see as a nice interior with nice finishes, but importantly, it's a ride that gets that 17-18mpg. For the same money I can buy a brand-new Class C 24' - 26'... I'm not looking for any particular answer on this, just if you can give me any advice that would help.

Hopefully these questions aren't too vague and generic to be answered...

Thanks in advance for any responses!