Hello world!
In desperate need of help: 3 mechanics, mucho dinero, and 0 progress.

I have a Chevy 454 in my 1989 Tiffin Allegro motorhome.
Here's what's going wrong:

Starting up: Instantly. Idling perfect.
Drives under 35 amazing.

Any speed over 45mph and the engine will stall, recover at 35mph, and maintain as long as I am below 45mph.
Additionally, if there are HILLS: it's game over. I'll stall at any hill, overpass, change in elevation. Downhill (if I don't accelerate) will stall me. Uphill if I push for the next gear.
I will be driving at a maintained speed, after stalling, the pedal gets no reaction until the engine "catches" again.


The mechanics have changed:
Fuel pump, fuel lines, fuel filter, inline fuel filter, vacuum hoses, adjusted carb (was put on backwards), cleaned the whole thing, changed air filters.
Fuel filters didn't show much sign of crud.

Every mechanic has said the same thing: Engine sounds great. Could pull the devil out of hell. Yet it can pull me at 45mph+ for more than 2 minutes.

What in the world is wrong?
Could there be water in the gas tank (RV sat for 5 years, just a guess by the previous owner)? Is it the transmission?
I did not ever drain the tank or clean it. I had assumed the mechanics would've check in. I was wrong.
Help?
I'm on the hunt for feedback from any knowledgable persons.

Can send pictures, videos, anything needed.

 

Welcome to the forum! Good job of listing lots of info; the more you tell, the more help you get around here.
Please clarify:
1. Biggest thing I read here is "sat for 5 years". Please go back 5 years - was it working fine at that time? If no was same problem happening then?
2. Did you try running it initially on 5 year old gas?
3. Have you drained/cleaned the gas tank?

 

Thanks for the response. I edited above but will add here as well.
Don't want to skew the results. Always hoping there is some silver bullet to save me headaches.

The previous owner told me it sat for 5 years at one point. I have not had the tank check, nor drained/cleaned.
I was even wondering if the wrong gas cap could result in some funky symptoms.

 

I once owned a Winnebago with a chevy P30 chassis. I had the manual for the P30 portion as well as the book of electrical.

If you have any use for it, let me know. If I am allowed to, I will post my email address for you.

I will be working with my book collection this weekend, If you are interested in it, and can use it, I will look for it.

I do not want anything for it. I would like it to go to someone who has a use for it. The motorhome I had had the 454 engine.

 

1. Check and confirm fuel pressure while under load.

Is this carb or TBI? You likely have a fuel pump in the tank and up front if it's carbed.