Have you ever tried to use a trip for navigation? It simply doesn't work, it's just a useless feature, that Garmin is unwilling to correct.
The only way to get multiple points route on 37xx is to use "Where to" multiple times. First time you set destination, then you add VIA points. Algorithm for adding VIA is stupid, it works only if you add VIA in revers order, from farthest to nearest.
You can prepare points for your route in Mapsource and than send them to favorites.