Although I have never tried this, I would suspect that 1500 rpm would be too low a engine speed to move your truck at 65-70mph. The engine would have to work to hard thus reducing your mpg. I have no doubt the cummins could do it but I do think it would be working hard.
I just took my RPM / MPH readings:
1500 rpm = 65 mph
1650-1700 = 70 mph
Yes, you are mostly right. The 1500 rpm / 65 mph only works on flat or maybe very slight upgrades, but nothing steep at all.
If the road steepens, if you are going only 65 mph, you will loose MPH in the top gear of the Gear Vendors, which is 5th - OVER. (that is, normal 5th + the Overdrive gear)
What I do in that situation is, to shift down to 4th, but leave the OD switch on, so it looks like I've shifted from 5th to 4th, but I'm really in 4th OVER, which is higher than 4th, but lower than 5th.
What you end up with is a gear between each gear, but it won't work under 20 mph, so you can use 2nd OVER, 3rd OVER, 4th OVER & 5th OVER - effectively 4 extra gears.
My truck, 2500 RAM CTD 4x4 runs on 19.5 12 ply tires - General LMT 400's that are 31" tall, and I have the high numerical rear end (not 4.11), PLUS the Gear Vendors Overdrive unit.
Keep in mind our trucks have the kinda week kneed 5th gear (my trans has the factory fix for the 5th gear 'nut falls off' issue), so with modest loads or for running emtpy, use 5th OVER, and you'll have better MPG, less revs & more relaxed cruising.
For best towing strength, use 4th OVER and it's bulletproof as you're straight thru in 4th, with the Gear Vendors taking all the abuse which it is totally built to take.
I'm not sure if you're still reading this thread, but hopefully others will benefit. Any questions, post here AND send me a PM so I know you posted & am prompted to respond to your query.
Take care, CTD Ram Truck fellows,