Eyeteeth,
I have to ask, because you didn't mention it, did you charge the new battery? New batteries are often somewhat discharged.
Also, check the battery connections, make sure they are tight (at both ends) and that they are not corroded.
If your house battery doesn't have a momentary contact switch, could it be on (connected to the house battery). If the house battery was low it could be sucking all the power from the engine battery. Or, it has already drained the engine battery.
I'm sure Griff will have a whole laundry list of items to check.
