I was in your shoes last year, so here is what I'd recommend,
-Call Cordovan Tires at 1-800-238-6469 - see web site:
-Ask about availibility of the
Powerking Premium Super BF34 (6 ply)
(As of last month, 6.70x15 are available (but not listed on the web site.)
-Ask for the name of a distributor in your area.
-Call distributor and ask for name of a local dealer who can order the tire.
(Neither the distributor or dealer will have ever heard of this tire, but who cares as long as the fitment is correct.)
I paid $59.00 for the each tire last year.
There are other bias plys around, but this is the easiest one to get. (Every tire dealer tries to sell a radial, they don't understand the advantages of the bias ply for a travel trailer.)