I used Hartford for about 3 years and their policy is an excellent one at competitive prices except for one thing: they do not offer a "full replacement value" option. When we purchased a new motorhome I wanted to have that coverage, so switched to a policy Iobtained through Explorer RV, an in-house agency for National Interstate. Have used them for the last 4 years and am perfectly satisfied.
Last I knew, Hartford did not distinguish between full and part-time use - essentially every policy is for full time. For that reason, seasonal users may be able to save money with a company that offers the ability to suspend coverage for portions of the year.
We also use Explorer RV, an in-house agency for National Interstate to insure our Blue Bird Wanderlodge and Jeep. We USED to use USAA but switched to Explorer RV when I was able to buy more coverage for half the price I was paying to USAA. Boy, that was a shock!!