Speaking from experience here the first generation ISB 5.9 98.5-2002 that is used in motorhomes is not the same as the one used in Dodge pickups and none of the Dodge/Cummins performance programmers will work on them.
While the above is true in that the aftermarket programers will normally not work on the earlier engines, Cummins themselves can change the power rating. We used this engine in some farm equipment we built and we could specify a certain HP rating. We just had to pay more to Cummins for more power. More power, sure enough, just pay us more! We actually installed the engines and then the Cummins rep would come by and up the HP rating to the level we needed and paid for. We had ours set at 315 HP.
But, as others have said, HP is only one consideration. The T1000 is rated at a maximum of 600 ft lbs of torque (I am currently putting one in my coach and that is the Allison spec sheet number). I would expect that the first limiting factor would be engine cooling as has been said. Cooling systems on rear engined rigs are probably the biggest issue that people have to deal with. Side radiators are better in many ways, but they also eat HP as they are hydraulically driven. Rear ones use less HP, but need to be kept clean.
My coach is currently a rear engined gas unit, but I am converting it to a Duramax/Allison. You will absolutely love the rear engined coach as you have found out. My friend with the same 5.9 engine in his coach absolutely loves the mileage he gets. You only go up hills a small portion of the time. I would leave it alone and be happy.
I have the same rig, it is a 35 ft . I know whatyou are saying about the power issue,I find that if you shift the trans manually and not let the tcm do it you will have more luck on the hills. You might not be the first to the top of the hill but the last at the fuel island. The mpg I find is great, we are heading to Alaska this year,we tow a Ford Ranger and you Don't even know it is there. The only issue that I know of is that they had a string of bad blocks I believe they were K blocks that were cracking, mine was replaced by cummins, just google cummins k blocks and they will explain everything. We love our rig and I am sure you will too. Happy rving.
I talked to Banks when I was considering upgrading the power on mine. Mine was a 230hp version. Banks said they could increase the horsepower by 60 hp for $2295.00. I passed. I replaced the injectors with 275 RV injectors ($600.00) and replaced the exhaust manifold with a two piece manifold Cummins said was required. The one piece stock one was cracked. I then talked the local Cummins dealers shop foreman into remapping my ECM to the 275 RV map. That cost one hour shop labor ($120.00). The power gain was noticeable.
Ours was 38' with one slide and we were full timers, so there were full cabinets, closets, basement and we did have a washer/dryer aboard. No problem cruising over the speed limit or climbing faster than the trucks.
Way back in the stone age, we had a 28' Airstream Excella, a 1982 front diesel, with a 180hp Isuzu engine on a chevy chassis, that would not go over 55mph and sometimes climbed mountains at 20mph, but always got us there and we always had fun.
Hope your offer is accepted. You'll love a DP that size.
If it is an ISB, the engine can be re-rated by the factory to a higher horsepower rating, if one was offered for that engine generation. For small changes all that may be required is reprogramming, and often no change in torque, just extension of the existing torque output to a higher rpm. Particularly in the 260-300 range.
A big part of the price of a factory re-rating is the higher cost of the warranty on the higher rating. I'm not sure how that gets applied to an out of warranty engine.
The aftermarket engine tuners sometimes do something similar, in other cases they increase torque output throughout the usable rpm range by dumping more fuel in and/or increasing boost. Changes like this often include new parts: injector pumps, injectors, turbochargers, intercoolers, intake and exhaust plumbing.
Banks retunes with plumbing replacements, turbocharger upgrades, in addition to changing fuel injection timing and volumes (whether electronic or mechanical). That makes the kits expensive, more than just re-programming a stock engine. Banks has kits for engine series going back to the mechanical B5.9 and C8.3, tuning specific to motorhomes.
Really big torque increases involving sustained outputs need might cooling system improvements. Modern ISB engines with high ratings in pickup trucks deal with cooling issues by dialing back power when things get to hot, you may not want to be doing it this way during a long pull in a heavier motorhome.
I've seen the B-series built to well over 600 HP, for competition. But you may not want to push that far, if you plan to travel more than 1/4 per trip.
We have a 1993 DP with the Cummins 5.9L mechanical engine. A previous owner installed a Banks Stinger on it as well as a Pacbrake and we are pretty pleased with the performance. Banks has data showing a HP increase from 230HP (stock) to 290HP with a big increase in torque as well.