FWIW, my starter relay went out also in my rig. The problem was an intermittent no-start. I suppose that the relay could have burnt points in it reducing the efficiency of the high power connection giving you a slow starter spin.
Sounds like what you have is low current to the starter, that's why I mentioned the ground strap being bad or having corrosion on the ends.
The other thing you can do is have the battery load tested at an auto store. Or if you have the switch which will use your house batteries for starting give that a try also. You may just have a bad cell in your (new) main battery or even low water if you can check it.
Cheap and easy first.