I can tell you for a fact you do need a new computer which you can find online used for $200 to $500. From a salvage yard they're 50 bucks from a Toyota T100 3.4 automatic but just about impossible to find.
Yes you need a new wiring harness which you can buy from toyonly or off-road solutions. Someone could manually modify a cable harness which I did but it took me several days and most mechanics wouldn't want to do that.
I believe toyonly and off-road? solutions would have to include those in their quoted price for a swamp..?
I get the feeling that your local shop that says they can do it has never done it before and don't really know what they're getting into. There are a lot of backyard mechanics like me that have done it themselves so it is possible. It was a common thing with 4runners but not so many have done it on on Toyota motorhomes.
You can Google "Toyota 3.0L to 3.4L engine swap"and find a lot of info.
EDWARD NELSON