Going into limp mode only when towing

[firedreamer]

OK folks need some help. Have owned a 2005 3500 Silverado dully for 3 years without problems. 163000 miles on the Duramax / Alison 1000 setup. The last month or so it has gone into limp mode but only when I start to tow our 5th wheel. It will reset if i pull over and turn it off for a min or two but I have only let this happen about 3 maybe 4 times. It will drive all day long and hundreds of miles at any speed without towing. So I had the error codes pulled and get the classic P0700 from the ECM and so I know that mean a internal trans error code and the local Alison shop will pull that code for me this coming Tue for $75 versus the Chevy dealers $100.
So here is my question, Is there anything that could possibly be wrong inside the trans or maybe the TCM needing to be re-flashed or replaced or something that could cause this without needing to do a complete rebuild? I just don't have the $ to do a complete rebuild right now, but something short of that , maybe. I don't mind spending the $75 if there's any chance they could find a cheap repair but if there's no chance at all short of rebuild and it's drive it for now without towing or rebuild I would rather not spend the $75 right now. THOUGHTS ?
Thanks for your suggestions

[Dsallach]

When was the last time you replaced the transmission fluid and filter. Sounds like the transmission is overheating. Do you have an auxiliary transmission cooler on the unit. Did the problem just start? Have you successfully towed other things with this truck? Yo7 did not say what size 5er you have. Are you exceeding the tow capacity rating of the truck.

[firedreamer]

Had trans serviced 2 years ago complete fluid and filters or 5er is max out at 14500# and have been towing it for 2+ years with out problem. last trip was over 15,000 miles. we are full timers I'm pretty careful to monitor trans temps and yes there is a ext trans and oil coolers so no issue there while driving i Believe the towing max for the truck is 16,000 # giving eng /gearing 4.10 /dully ....etc

[Arch Hoagland]

When they did the transmission service did they do a power flush or a drain/refill and then drain/refill again?

[mike toon]

try driving with the electrical plug disconnected from the tow vehicle.

[firedreamer]

OK, here's the codes i got today from the trans shop ....
P0670 glow plug control circuit ( thats been in the History for awhile)
P1093 fuel pressure low while enrichment returned

the tech said he cleared the codes and then drove the truck and was able to recreate the problem when he "got on it pretty good." Then he got these codes. So I'm thinking it's more likely a fuel pump problem? yes /no? It's going into Chevy dealer tomorrow for them to tell me whats wrong and cost to repair.

[firedreamer]

A bad fuel pressure regulator, thats what dealer found so pretty easy fix when they get the part. may take a few days tho GM does not have one but some other dealers in the region do. thanks for all your input folks