Yes, do adjust the valves - that along with timing are the two things that can kill a VW engine.
What is the engine number? I have tables to show the correct timing assuming the original distributor. There are websites with the info too.
83,000 miles is probably close to worn out for a 1600 (upright) engine in a camper. Does it ooze oil from much of the engine or is it still reasonably oil-tight? Does it use rather than lose oil? That's a good first check on the state of an old VW engine.If everything else seems OK, and you're sure you aren't expecting too much of a 1500kg bus that has an engine which has the power of a small Polo these days, it may be worth getting the compression checked. Turn the engine over by hand and listen for wheezing in the exhaust or (less likely, the carb) in case it's a cracked head or burnt valve. Cracks are very likely in a bus engine, but need not be a major problem.
You might also check fuel flows well from the fuel pump outlet when cranking (run into a container clear of the engine and sparks), but I would expect some sort of misfire if the carb was being starved of fuel at higher speeds.