This would be your first port of call. Failing that,then Tvs or a similar firm could make one up,though it may mean changing the zip. Gazebos are not always acceptable on sites,but porch awnings can be used with a bit of ingenuity.
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It's for a Raclet Herald/Jade but may well fit a Marathon, could be worth asking (i'm not the seller, just trying to help).
I use my TT mostly without an awning unless I'm camping for more than 3 days or so. You could use a gazebo or attach a tarp to the front with a couple of poles if the weather is good