Carrie - the train I'm looking at departs at 6.13pm on a Tuesday - do you think that might be a bit busier? I'd probably aim to leave DLP by 3.45.
If I look at a Tuesday in October (as the RATP itinerary search doesn't go any further) for a depart from MLV-Chessy around 3.45 I get the following
<table summary="Description complète de toutes les étapes de litinéraire"><tbody><tr class="even"><th scope="row">
</th> <td class="step">
aller jusquà
Marne-la-Vallee Chessy RER </td> <td class="horaire"> 15<abbr title="heures">h</abbr>48 ; </td> <td class="duree"> 3 <abbr title="minute">min</abbr></td></tr></tbody></table>
<table summary="Description complète de toutes les étapes de litinéraire"><tbody><tr><td class="etapes" scope="row">
</td> <td> depuis
Marne-la-Vallee Chessydirection
Poissyjusquà
Chatelet-Les Halles </td> <td> de 15<abbr title="heures">h</abbr>51à 16<abbr title="heures">h</abbr>31 </td> <td> 40 <abbr title="minute">min</abbr> </td> </tr> <tr class="even"> <th scope="row">
</th> <td class="step">
correspondance </td> <td class="horaire">
</td> <td class="duree"> 1 <abbr title="minute">min</abbr> </td> </tr> <tr> <td class="etapes" scope="row">
</td> <td> depuis
Chatelet-Les Hallesdirection
Mitry-Clayejusquà
Gare du Nord </td> <td> de 16<abbr title="heures">h</abbr>35à 16<abbr title="heures">h</abbr>38 </td> <td> 3 <abbr title="minute">min</abbr> </td> </tr> <tr class="even end"> <th scope="row">
</th> <td class="step">
aller jusquà
Gare Du Nord (RER), Paris </td> <td>
</td> <td> 3 <abbr title="minute">min</abbr></td></tr></tbody></table>
So 53 minutes all together and then the walk up 2 escalators across the concourse and up the other escalator to the Eurostar floor and join the queue
You will be out of rush hour as it starts from 17.00 and you will be there in loads of time