Found a local independent dealer who sells Goodyear - must be a franchisee since he has a Goodyear sign, but is not listed on the Goodyear site - but he can get Marathons (225/75 R15, load range D) for $92, plus balance and stems (metal!). Mounting is free. Stems are $3 - most places I looked charged $5. Probably be a week or two before I have time to get in there, but in plenty of time before my long Labor Day weekend trip - where I was most concerned about my existing tires surviving.
The information I say says the load range D tire in this size is rated at about 2400+ lbs. That should be plenty since my 3124 weighs 4240 lbs (weighed it pretty much ready to camp - no water, but batteries, propane, and all options in place).
I'll keep you all posted on progress. And thanks for all the advice and direction.
Alan