Congrats and welcome to the forum. Our Forester MBS just turned 3 years old and we've put 60,000 miles on it. I have learned a ton of good info from our fellow forum members!
You should be aware that this modern Mercedes diesel engine requires specific oil to protect the diesel particulate filter in the exhaust system. It does NOT use normal diesel engine oil. Ignoring this could lead to a very expensive repair. I do my own oil changes but it would not surprise me if your local oil change shop does NOT know about the special oil needs nor stock the right oil.
You can use Amsoil ESP Euro Oil or Mobil 1 ESP. (ESP stands for Emissions System Protection). The Mobil 1 often goes on sale on NAPA's website for about 40% off. Even if you have a shop do it, you could by it on sale and bring it to your mechanic to save money.
For engine fluids, Mercedes gives a spec number that when Googled produces a long list of approved fluids. Unfortunately, most of the brands are European and not easily found in the US. I did a lot of research and created this lube chart which I've attached for your convenience.
Also very important to know is that they prohibit any diesel fuel that's more than 5% biofuel, known as B5. You will sometimes see B20 diesel for sale at a much lower price but it should NOT be used per the owners manual. Some skeptics may reply here so I have also attached the Mercedes brochure on diesel fuel. It includes photos of the damaging effects of using B20 in their engines. You can make up your own mind......
This modern German engine is very high tech so you should read and follow the scheduled maintenance for the engine. I recommend finding a repair shop that specialists in Mercedes. The one i use has the same special training and equipment but charges at least 1/3 less than the dealer. If you're not a do it yourself like me, by using a Mercedes specialty shop you won't have to worry about all that Mercedes spec gobbly-gook that i had to figure out. They will already know all that and do it right. I also attached my own maintenance chart which I created from the manual.
Now for a word on Diesel Exhaust Fluid (DEF) usage. Every diesel brand and model of engine uses this stuff at a different rate, so you should only pay attention to what the owners of an MBS like us say. My indicator light first came on at 7,800 miles. My MBS uses it a gallon approximately every 2,500 miles which seems to be typical for an MBS. At that rate you can still drive hundreds of miles before the notices get more aggressive. So just add some at your next opportunity but don't worry.
As for the RV part, I've had LOTS of rain leaks so can your eyes open for water, stains on the ceiling and walls, etc. Get them fixed while you still have the warranty. Also had occasional trouble with the slide and other items.
Many of us here have installed and will swear by a holding tank vent that creates a air flow out of the tank to remove odors. It works really well!
I might mention that the Visa credit card from Fort Knox credit union rebates 5% for all fuel purchases. Many others may chime in about their 5 cent discounts, but this is FIVE PERCENT
! Diesel is now over $3 here in southern OR (and FAR higher in CA), so that's a 15 cent per gallon rebate.
And I'm standing on the mountaintop and shouting praise to the SeeLevel tank sensor system. It's the best mod I have done to improve our RV experience! Thanks again and again to whoever posted about it when I first saw it.