Having come from owning an SMB for 8yrs, pretty much everything in the ER is more difficult and expensive - oil changes on the 6.7 diesel are like $200-300, and if you want to keep it in best possible condition the recommended is about every 3-4k miles, so you can do the math. The 41" MPTs and Hutchinson rims are they're own little project as well - you can't just roll into your local tire shop to have them balanced, rotated...unless you want to roll the dice and then risk some resulting issue in a place where help or a tow might be unavailable. Also 95% of shops won't touch a 41" tire on a beadlock rim and most won't have the rack required to get a 16-18k lbs rig up, so you likely will need a truck tire or RV shop. So be prepared to qualify up front the shop/tech right when you walk in the door to save yourself and them a bunch of wasted time and energy. There are a few owners who have the time/skill to do rotations, etc themselves, but the Hutchinson's take extra time and have some unique quirks you need to be aware of (O-rings, leaky bolts, adapters, etc.). However, this adds cost compared to standard rig you can take to any tire shop. The only other system on the rig that is somewhat unique, but critical to operation, is the Accuair suspension system. I carry 1 extra airbag and tank pressure sensor, as those have failed in the field before. I've had no issues, but a few other owners have so better safe than sorry.
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EDIT: Also, join the ER owners forum and spend some time there. Several owners have posted about ongoing costs.