Class A with the Cummins 275HP engine?

Peak torque on your engine is probably in the range of 1,600 RPM. Pulling a load below that RPM is lugging the engine and not good for it. Have you tried downshifting manually when your RPM has dropped to 1,800 or 1,900 RPM and running in the 2,300 to 2,500 RPM range when pulling those hills? That is the general operating strategy I use with my 300 HP, 600 ft lb torque at 1,600 RPM Cummins ISB/5.9 powered coach. Works well for me, give it a try.

google is your friend.
Are you the one who install that engine?

I have that engine in my 2001 Kountry Star by Neumar with the Allyson 5 speed tranny. Mine shifts from 5th to 4th at 55 mph, and 4th to 3rd around 45 mph. I say this because sometimes I do manually drop it into 3rd at 45 to keep the RPMs up. I try to keep my RPMS above 1500 when pulling hills.
Come to think of it, I had a 99 International with the 270 engine (not Navstar) and 6 speed Allyson. It would backshift to 5th at 55 and 4th at 45 with RPMs around 1500 also.
IMHO, don't wait for the engine to lug before downshifting. You are better off to keep your RPMs up by backshifting manually when you can feel the engine lugging.
When I am approaching a crest and my MPH hit 46, I upshift and then let the tranny take over. Usually, by then my RPMs are over 1800. Also actually better fuel consumption by not lugging.

I drive mine just like Fleet Man said and agree this works best. it also keeps temps. down on long grades in hot weather.

Mine with the 6 speed Allison in Mode shifts to 6th at 52 MPH and at times will drop to 45 MPH before going back to 5th.

But when in hilly ares I take it out of Mode and downshifts are sooner.

Then if it starts hunting between gears on a long climb I manual downshift and keep RPM around 2,100-2,200.

Thanks guys, this build is just let me say different. The trans started life as a 5 speed 1000 Allison trans. The people that did the build 52,000 miles ago. Put the Sun Coast 6th gear kit in the trans. So with that it does not allow me to downshift from 6th to 5th. I think for some reason this kit is making it go from 6th gear down to 4th. Thus skipping 5th gear. Going to get it back to Beans Diesel east OF Nashville Tn. And see if they can re-tune the trans. As the paper work shows they did the trans wiring and tune when it was Built.
If for some reason they can't get the trans do shift right. Will sell this one and put at 06 or above with factory tune 6 speed Allison 1000 in it. As the ones that are running that trans and a box made in South Ms.Along with the factory tune. Are finding it's working perfect. But that's $6000 more to get it working I hope.
Engine does not feel like its lugging even at 1400 RPM's and 45 MPH. Truck is pulling good. But as you know your building higher EGT's when doing that. I have driven over the road. For years and even back around 91 the Freightliner I was driving back then. It pulled its best at 1150 to 1200 RPM's. Everyone back then were talking about lugging the Detroit 60 series 12.7 but that is where it pulled best at.
Have had 3 guys that have worked on these engines of years, tell me its not hurting it at 1400. But I for one would sure feel better at around 1600.
Thanks guys for the info, and I have searched for the above info over the last 2 years or more. And its hard to find real world info. I found lots of info on the engines in Dodge trucks. But as you know lots of difference between our 275HP and there 230HP or so and less Torque.
Pete

Keytag1 No the original owner did the build at around 20,000 miles on the truck. Second time he blew the engine. Chevy black listed the truck so his warrant was gone. He forgot to unhook the big tune on it. Thinking he had around 500HP out of the Duramax both time it ate a piston.
They were pulling a 45ft inclosed trailer full of Custom motorcycles around the country with it. Over grossed and high HP got the best of it for him.

So he bought a 10,000 mile 2000 5.9 275HP engine out of a Diesel pusher. IT had burned from the Fridge. So engine sat for a few years. Then he took around 1 year getting the build done. Destroke build the trans box for him. That from what I found out was the first one they did for the 5.9/Allison for the chevy. Hundreds of hours later and 3 days of someone from cummins main office at the shop. They got it to run right. Pulls great and not bad on Fuel when I drive it right. If I keep it around 65 it will get 12 MPG. IF I run it had 8 MPG. Grossing most times close to 30,000 lbs. 6th gear 0.61 ratio, rearend ratio of 5.13 and 19.5 tires lets it run 70 MPH at 2150.
Also have all the paper work on what work they did parts for the build.

If I don't get the engine and trans to work out. And don't sell it and buy a Semi to pull our camper. I'm thinking a newer 6.7 cummins and trans out of a Ram may find its way into it. That or a M11 cummins.
Pete