We too have Coach-Net ERS and have for years. As we mentioned in another thread, they have a 24/7 RV Technical Assistance Hotline which might be useful to you as a new owner of an RV. =click here
for their motorhome coverage description.
Although we've had GMAC insurance coverage for several years (not through Good Sam, however), we haven't yet seen a premium increase. Haven't had any claims either, however. Our philosophy of insurance (and not all agree) is to have coverage for catastrophic events. We prefer to "insure" ourselves for the small stuff. So we have never had a claim on any vehicle insurance for decades. Therefore, our deductibles are very high and we don't have a lot of frills added to the policy.
For the reason mentioned above, we use an agent to shop for us as we usually want to go with the least expensive alternative. To us, insurance is insurance (again, not all will agree). Going though agencies that specialize in RV insurance, we've had National Interstate for quite a few years and now have had GMAC for the past several years.
Miller Insurance in Lake Oswego Oregon is a popular agency amongst full-time RVers that specializes in RV insurance and we've used them =click here
. We now use Poliseek in California =click here
. Being full-time, we want to use an agent who not only understands the special needs of RVers but also one who understands the special circumstances of using a Montana LLC (I won't explain what that is here as I'm positive I'll get flamed extensively). But both Miller and Poliseek understand both those special circumstances so they can narrow down a carrier that will be suitable for us. Again, agencies are businesses that shop for a carrier and arrange coverage for you. They are not actual insurance carriers/companies. You don't have to use an agency should you not desire. I only mention it as an alternative and many do not like to use agencies.
As for towing a car behind you rig, iRV2 has a special forum for those questions so when you get a chance, post your inquiries there:
Toads and Motorhome Related Towing
Good luck and keep asking questions.