If I had a diabetic dog I think I'd either board it with my vet or I'd find a certified vet tech who did dog sitting and who had fabulous references, and I'd have him/her come at least three times a day, and preferably four times a day. I think for me it would be a hard call between those two--the dog would probably be more stressed at the vet, but would have someone around most/all the time (depending on staffing overnight), and of course a vet readily available much of the time. The dog would probably be more comfortable/less stressed at home, but much more time alone.
I would NOT do a non-vet affiliated boarding facility. Nope, nope, nope. I barely trust even the best ones for healthy dogs.