Timing Curve on 360 in an RV

If you have headers;
and an exhaust leak at the head,
this can cause the "backfires" you speak of which are actually after-fires, as a rich mixture leaving the combustion chamber continues to burn in the headers. Starting the fire earlier as you are doing, helps to alleviate the popping but is the wrong solution.
The better solution is to lean it out, and fix the exhaust leak, and put the timing back to where it belongs.

Log manifolds do not work like headers and do not cause this kind of problem. They are generally, under pressure almost all the time, more so if afterfire is occuring. But they can do a different thing. At the tail-end of the exhaust stroke the intake valve is already beginning to open, so they are both simultaneously open for several degrees; typically more than 30*, and less than 50* for an RV -type cam. If the log has a higher pressure than the intake, which it always will have in a rich-running engine, AND the cylinder also is at a higher pressure than the intake, which it always will be with logs;
then the pressure in the cylinder will go to where ever the lower pressure is. Often this is the intake manifold .... so now you have a pop in the intake, a genuine "backfire".
However this kind of problem goes away with increased rpm, as there is no longer enough time for the exhaust to behave this way. The cure for this is a free-flowing exhaust, a leaner mixture, and proper ignition timing.