May be a bug problem
Bugs love to plug up the drain hoses by building nests. On a prior SOB trailer, I solved the problem by slipping a plastic strainer meant for a paint spray gun over the end of the hose. I had to open up the hole in the strainer a bit so that it wouldn't pinch the hose shut. I never had a problem after that. The strainers cost only pennies and can be found at Sears, HD, and the like.
John W. Irwin
2005 Classic 28 "Sabre-Dog III"
2013 Silverado 2500HD Duramax/Allison LTZ
WBCCI #9632, TAC TX-10