When our '66 Safari did the same thing, we found the drain line under the trailer completely stopped up with dirt dauber nests (unknown age). The trailer had been sitting for 6 years since the PO had used it.
At least it's a cheap thing to consider first, if you live in, or have been to dirt dauber country. Those darn wasps will build in any tubular structure, since it saves them time in building tubes out of mud.
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