That's great. On Steel Soldiers you will see the general advice for getting a new CUCV is to change the fluids, anything rubber (seals, belts etc) and definitely do the Doghead Relay Conversion. This is swapping the original GM starter relay, under the dash, for a more robust starter solenoid to avoid the starter running away and frying wiring and batteries. Just search Doghead Relay and there is a simple set of instructions with photos. I have found a UK site on eBay which does a solenoid for £9.99 and it took me about an hour and a half to fit it - and I am no mechanic.
The other thing is to watch for broken or detached wiring grounds (earths) of which there are loads on CUCVs. Again SS will tell you where to locate them. I have a 1985 M1009 Blazer.
Re batteries - just look up in the TM the correct way (and sequence) they should be connected - my truck helpfully has numbers painted on the connectors to match those on the batteries.
I expect your problems are a combination of you and your mechanic being unfamiliar with how these trucks are set up, likely a cable has been connected wrong or a loose wire or a blown fuse - again under the dash and they are just standard fuses. Remember CUCVs are mass-produced commercial vehicle with parts common to many GM and others products and they are well supported in the US, Europe and by American car parts suppliers in the UK.
Keep me posted.
Steve