I wouldn't give up on this one just yet.
I'm thinking 'steal' here... none of those problems are insurmountable. Take a starter battery with you (and a jump pack.) None of those things are irreparable if the drivetrain is good. Take $2500 cash with you. I wouldn't go any higher than that for it under those circumstances... but if your mechanic friend pronounces it 'drivable' you may have a real gem there.
The difference between the asking price and $2500 will cover just about any contingency in the RV portion of the van. Just make sure it's been winterized, either with pink antifreeze or by having the air blown out of the lines. If there's a problem with the water heater tank and it's been frozen, you'll likely be able to see it as it'll have a jagged tear in the tank about 6" long somewhere. The water lines are harder to see, and may be more difficult to repair; but not impossible if necessary. The odds are that the fridge will work, the water heater, stove and furnace will all work, or can be made to work with very little to be done to them. You'll likely need to figure in the cost of new tires, complete fluid changes... maybe brakes and some of the rubber lines... brake lines, hoses, belts etc. but you'll likely need to do those at $4100 anyway.
If he doesn't accept your offer, give him your name and number and tell him the offer stands when he's ready. And walk away. You might be surprised when he calls you in a week or two and tells you to come and get your van.
The big thing is to make sure that the engine and trans are sound... as they're big ticket items that may be outside your budget. But I've gotta tell you I think that's a pretty sweet van... and potentially a sweet deal.