Is there anyway that sombody tell what is the length for ground rods for trailers ,someone told it should be like 8 feet long ,but I find it difficuilt to get that length of the ground rod under the ground ,is there any other way,another person told me that I can connect three shorter grounding rods together to act as an 8feet one

This is dangerous but sometimes done. The ground rod uses the moisture in the ground to do its job. Not long ago during a drought we were digging with a backhoe and noticed that even in clay we had powder at 4 feet. You can rent a tool that will drive the ground rod. It is like a hammer drill without a drilling action. All good electricians own one.