It's probably a bad battery - especially if it's Chinese made.
When I opened my Chinese made rechargeable fan I found the battery puffed up (a lithium polymer battery) and the amp hours much less than claimed on their advertising. Dead battery.
The manufacturer either lied or their battery supplier cheated them.
The lipo batteries will die if their voltage drops below a certain point. I suspect that, over time, they self-discharge to the point where they won't take a charge.
True, but at higher elevations, fuel burns slower, negating the need for higher octane. Go to Denver, or higher, and 85 octane will run just fine in most gas vehicles, unless they're of European origin.
Fuel is indeed getting pricey, but no more than most other things. My grandsons, working at McDonalds, work less to drive 100 miles than I did at their age. But, my '68 Barracuda 6 cylinder only got 17 mpg. Fuel today is a relative bargain.
Around here right now anyway, midgrade or premium gas is more than diesel. The new Ford 7.3 gas engine calls for 87 octane which in high elevation states would be midgrade. Fuel in general is getting pricey.
Not sure you are really considering "engine" noise, but perhaps engine noise you hear in the drivers seat. Yes DPs will produce less noise in the drivers seat due to location of the engine but the engines themselves, well most diesels will be noisier esp under hard accelaration. Unless you will be towing something heavy, there is no reason to dismiss gas engines. Those chassis are designed for that service and they are less expensive for a Class C properly outfitted.