When I was in elementary school once a week one came in packed with books (book mobile) probably around 1965 but it was brown instead of blue. It has to be early 60's
The plate says ‘70. RVs back in the day usually didn’t have the most up-to-date mechanicals and of course used truck engines. The 413 wedge was used in cars until 65, but still available in (mostly commercial) trucks until 79, and RVs would use a heavy truck chassis so it seems reasonable that it’s a 70.
There are several Travcos on You Tube that have been Cummins swapped. Travco was one of the nicer motorhomes back in the day. I'd like to have one to restomod and Cummins swap for our one lap of America when I retire, but I have too many other projects and too little time!