Tim I have the same problem with my 1993 Southwind on a P-30 chassis. My 454 has 78xxx miles and has run very well up until it started doing, I believe, what yours is doing.
I have tested the MAP sensor, replaced the TPS (throttle position sensor) - I replaced that because the OBD1 system on mine said the TPS had low voltage. Replacing it did no good.
Engine has the DI type distributor. It is not an HEI like its predecessors, it has the ignition coil mounted off to the left on the intake manifold (changed that with a spare coil I had) no change there.
The plug wires are the MSD street Fire 8mm ones and the spark plugs seem to be firing. I installed headers last year on the engine. Good thing about headers is that you can test the temperature on each one of the tubes while the engine is running to see if they are all the same temp and if not then likely one of the spark plugs are bad.
I have not changed the coolant temp sensor on the front of the intake manifold but the engine idles like it should, high when cold and then idles down when warm.
I tested the EGR valve and the vacuum ports on the throttle body and they appear to be good. I replaced the in tank fuel pump last month thinking it was fuel starvation while going up hills at WOT(wide open throttle). The fuel pressure with the old pump was low showing around 8 psi on the pressure gage that I permanently mounted 3 inches from the throttle body on the pressure line.
I have a timing light that I bought just for this engine and checked the ignition timing ( with the brown wire disconnected on the distributor) and set it to around 10 btdc. One thing I have not checked yet is the timing advance with the brown wire connect while revving the engine to around 2500 rpm. Will need to get help from my better half for that to press the gas pedal (or rather the air control pedal).
I plan on sending the two fuel injectors to a place called "Mr. Injector" to be cleaned and flow matched.
I'm not really sure about your distributor but mine does not have a vacuum advance or the mechanical advance on it. I did remove it from the engine last night to check the shaft for any play and it seems to be in good condition.
I am at a loss here. I really don't want to take the motorhome to a shop just to hear that they cannot find the problem and then be charged for the time they spent looking at it. Being a vet on disability I can't go out and keep throwing money at this thing anymore.
We have not been able to use the coach much this year because of this problem. Its very frustrating not being able to enjoy the coach because I can't find the problem to what is causing it to fall on its face while going up hills at wot or close to wide open throttle. Like Tim when it starts to cut out and run bad if I back off the pedal it will resume acceptable running but go above that spot and, well, it is not good. Especially if towing the Mini Cooper, its even worse. As long as there are no hills or other vehicles to pass the engine seems to run very well.
This is my first time posting on here guys.
Hope I did it right and made sense to all. Thanks.
John MSGT Ret USAF