Anywhere heavy loads are designed for install on outside walls - steel plates are built into the wall for additional support. Usually can be found around edges of cabinets, hand rails, seat belt mounts etc. Take strong magnet and test area. Also could be hitting aluminum tube that is built into wall. On mornings when dew is on sides of coach you will see outlines of the aluminum studs. Don't think you should be hitting anything hard on interior wall. BE VERY CAREFUL drilling holes - study area first to see if plumbing or electrical stuff might be there.
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