If you trade for a bug, you will get a little better gas milage, but you give up a lot of room and portable home away from home. Your gas mileage won't really increase that much. I've got a 1600 type 1 engine in Homer the Superbus . . . I usually average anywhere from 19 to 22 MPG, depending on how and where I drive. With the same engine in a bug I get about 28 to 30 MPG. That looks like to me at best about a 1/3 savings on gas by going from a bus to a bug. If you're spending $70 now, you're probably going to save less than $25 a week.
If you go to a bug you are also going to add a few regular maintneace items . . . Your '77 should have hydraulic lifters. These make regular valve adjustments unnecessary. The disc brakes up front don't require adjustments like a bug's front drum brakes do. One advantage to a bug is that parts are generally easier to find and almost always cheaper than their bus counterparts.
If you really want high reliability and much better gas milage, you could buy a new Japanese gas/electric hybrid car. You would cut your gas bill to a fraction of what it is now. Of course you gott'a make payments and carry full coverage insurance. And if it does break down you'll probably have to pay a dealership technician several hundreds of dollars to hook it to the computer and tell you that it needs several hundreds of dollars worth of parts. Those little hybrids don't do very well on rough back roads going to a camp site either. And they are kind'a cramped if you do get to the camp site . . .
What I'd do is look around and try to find a cheap bug and buy it outright. You can probably find a decent driver for about what you might have to pay for a good late bus transaxle . . . if you're planning on replacing it just because it's old and it could fail, I'm betting that you haven't priced one recently have you? Anyway, if KEEP the campmobile and add a bug to your stable, you'll have the option of getting better milage when you don't need the room in the bus, but you'll still have the bus if you do need the room or want to go camping. And if one breaks down, you've got a back-up . . . and you don't have to have an engineering degree and a 29 Gigahertz Pentium 53 with a bazillion megs of RAM to figure out what's wrong when something does go wrong. You also won't have car payment. OK, you will have to pay a little more on insurance to carry the second car . . . but that twenty-something dollars worth of gas you save each week could pay for that . . .
OK, with all that out of the way, I'm going to give you some good advice. You don't need to doubt! You can rest assured that your bus will break down on you sooner or later. But, you can also rest assured that it will probably be easy to diagnose and, relitive to a new car, be cheap to fix. And no matter where you are, when you break down in a VW bus, friends you never knew you had will show up to save the day. Some day when I have the time I'll tell you about driving 150 away from home stuck in 4th gear, and then blowing an engine . . . the only tool I had with me was a screwdriver and I had $60 cash. The next day I drove home with a new engine driving all 4 gears.