A valve spring problem, as Bill mentioned, is something I had considered. Your definition of "under load" may need better definition. Popping due to spring problems happens at high RPM, and loudest "with the hammer down".
I'd like to be told positively that it's Carb or it's TBI. A "1988 motorhome" can be on a 1988 chassis, but could easily be 1987, or occasionally even earlier.
A 454 wound out, climbing a steep grade, needs 25 gallons/hour. Fuel pump, fuel filter, fuel return line... I like the possibility of an intake air leak, leaning it out. Could be EGR like Nb2 said, intake manifold gasket, carb/TBI gasket...
Did you check for clogged fuel filter? How about cap ,wires and rotor? Seem to recall some HEI rotors burning through the carbon button. Is it running hotter than usual? Another thing, are they putting corn juice, ethanol, in your gas? Just went through a Rochester 2 barrel twice in 2 years due to ethanol, ate up the float.
Got one of these, timing marks in the bottom, distributor is on the top, cylinder #8 is where your you hook up your timing light to, also need some one on top to adjust it. Not sure about your 88, mine has the old HEI distributor with the vacuum advance, that has to be disabled first. Not even sure how this thing ran when I got it, was 30 degree off.
Don't even want to think about water in the tank, sure had a lot of rain up north, stalled in the rain in the middle of nowhere. Good I also have a electric fuel pump, had to crawl under, remove a fuel line, have my wife my switch on the pump, spilled that water on the side of the road until gas came out. This got us going again. Bot tanks had water, just did the front one, rear one when we go home.
Sure can't put E10 in these things, never heard about this stuff when these things were made will also mess up your generator if you have one. Sure messed up my 88 Supra, for awhile, this is all we could get.
Reminds me to order a conventional oil dipstick, mine needs new exhaust manifold gaskets, no way to remove the one in there, goes clear forward to the top of the radiator for convenience, was put in before they built a motorhome on top of it. Just have to pop off the engine cover to check it.
Craigslist around here are loaded with these things can't even give these things away, RV dealers around here charge 125 bucks an hour, campsites are 30-40 bucks per night, but we go in the wilderness. Purchased our 11 years ago, and if I deduct what the last guy put into with all brand new parts, paid only 1,500 bucks for it, only had 39K miles on it. Sit around more than they are used, had to redo the entire engine electrical system.
For one thing, had three brand new batteries, negative battery terminals connected together, and of all places tied to an engine shield ground, made a new cable to tie that to the engine block, then another directly to the chassis. 12 volts for that firewall connector using all those unplated brass Packard Electric spades ran clear down to the starter terminal and back again, when it was only a foot away, had to clean up that corroded mess.
Could have spent a couple bucks more and purchased a newer one, but with my kids and own crap, had plenty of EFI stuff around here to drive me nuts. With this thing, battery, fuse, ignition switch, firewall, just one wire goes to the HEI, simle, not a million transistor for that same route. Pulled the HEI, top bearing is always worn, pressed in a new one I had laying around, and properly soldered the two wires to the trigger coil, GM never did this right. The list goes on.
Do like having that Owen generator, sure got a ton of rain here last fall, electric power went out for several hours, just ran an extension cord to my basement sump pump. Also handy to have when our congress picked on that 3/8" high pilot light, never lost natural gas, but with a 75 year old electrical infrastructure, these failures are common, can't even switch your furnace on to least get some heat. Don't have to run that generator constantly, just enough to keep my basement dry and that we don't freeze to death.