I have a motormaster eliminator powe box 1200

replace batteries

Battery keeps dying and radio defaultig to am

Sounds like a ground loop. Is the noise present with NO inputs?? Or?

Replace thermal sw's

if there is a car fuse, under the cover, remove it.
most of the time, it is only use to activate the LOW Battery alarm.
Put the fuse back, when fully charge.

Get out the measuring tape. As long as it will physically fit inside the van it will fit. Pay very close attention to height because if there are posts on top of the battery you don't want them hitting metal. That will cause a short are ruin the battery. If the height is close then use a piece of rubber or plastic to shield the top of the battery. Hope this helps.

Inverter run on batteries sir, and if these batteries are not charged, the run time {the number of hours it will work} reduces.
S0, you will need to connect the inverter to a power source.Theses will allow your batteries to be charged and enable it to run for a long period of time.
Make sure your batteries are still OK.i.e working properly

Three years is pushing the life of the batteries. Have them checked where your type of battery is sold.

Recommend (1) open the inverter (with battery power disconnected) - and inspect incoming cable between the power switch and the battery cable entrance. Possible that a wire overheated and shorted on the input cable.
Next (2) if problem cannot be located, go to http://search.harborfreight.com/cpisearch/web/search.do?keyword=inverter and replace it with at 2000 watt inverter for 130 bucks.

most of the time, those connections are on the bottom-right of the motherboard. Look carefully for markings or words.

Most often its like this
reset HDD POW
o o o o o o o o
o o o o o o o o
PW/SW

or somtimes its above like this


reset hdd pow
pw/sw

o o o o o o
o o o o o o

LOOK AT THE MARKINGS. HOOKING IT UP WRONG MAY CAUSE PROBLEMS

hookups are usually like this


o o o o o o
o o o o [o o]
PW/SW
hope this helps