Preparation is crucial. Clean the surfaces to be sealed first. There are several different brands of sealants you can use, (Google search for these) and then practice application and spreading (with wet finger). Be sure to trim very little of the tube tip as you want a small amount and width of sealant applied.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7