They are correct, generally speaking. However, if they made a promise to you that the lot is secure, then you may have a case. But I doubt they would just roll over and pay you and set a precedence. You'd have to go to small claims court.
Generally speaking when your RV gets broken into the only people responsible for the damage are you and the people who stole your stuff. The owner of a storage lot or a repair facility won't be responsible, unless you can prove some type of negligence or a breach of a contract on their part. For example if they left the motorhome unlocked, or they promised you there would be a security guard at the lot 24 hours a day and there wasn't, or if one of their own employees broke into your motorhome or conspired with the people who did. Just saying "the lot is secure" probably won't get you there.