Idog, I've been having the same problem......
I contacted a private owner through an rv listing company and asked about renting a new diesel pusher for 3 ~ 4 weeks. The owner assured me that "you shouldn't have any problem getting a binder; most companies tell you they don't do it but they do". He told me "I'll give you the information you have to know to tell them in order to get an umbrella policy binder ", when he needed to have the verification.
Well, we spent a good many days/hours planning a trip through the Southwest, called him, reserved the motor home and (without even considering it might be a problem) sent him the required deposit. He telephoned to tell us he got it and that he was including a number of things not normally included and wasn't going to charge me mileage or generator costs since I was renting it for a month during the off season.
Then, we started the "hunt" for a binder. Well, all the companies you mentioned told us the same as they told you. I called yesterday and left a message informing the owner that I needed his help and the information he promised me; he didn't call me back last night, so here's hoping he will. I went on a "scavenger hunt" on the 'Net and found out that, supposedly, Farmers Insurance will do it, American Families Insurance(? new to me) does it, and State Farm will do it for free, provided you're an existing policy holder. Also, if you're a Vet, USAA will do it for you also (according to agents from 2 rental companies).
I talked to the local State Farm agent and he explained that, were I a policyholder, they'd simply add it onto my existing auto policy and cover the motor home for just the rental time. However, there's another insurance caveat that "loss of use" is not covered that way. That means, should I wreck it and it would be in the repair shop for a month, I'd have to pay the entire rental fees for the month that it couldn't be rented it, should the owner demand it. The agent told me this(apologetically) because they have an Un-owned Motor Coverage (UNOC) that would cover loss of use, but he'd have to charge me a fee for UNOC. It could be added onto my policy for just the time I had the motor home rented so it "wouldn't be much".
I haven't had time to contact the other companies but I hope to do it before the Virtual Campfire tonight. I hope this'll help you.