a 12V 7Ah battery is 84Wh energy capacity - not all of which us available - unless you want to send your batteries to an early grave. Say 30% discharged only - that means only 30Wh is a sensibly usable.
If your heater uses 300W (doubtful as the varrox was only about 150W) would mean one fully charged battery (in ‘as new’ condition) would last just 6 minutes before needing to be recharged.
Just some simple physics - and simple maths to calculate it out. Does that help you make a decision?
The answer is no, if you want your batteries to last. Incidentally, automotive batteries (this includes motorcycles) are not manufactured to be discharged to any great extent as they will only last a very limited number of ‘full discharge cycle equivalents’. They are normally returned to full charge soon after operating the starter motor and power for ancillaries is derived from the automotive generator while the engine is running.
I think you need to investigate the power consumption/supply, as well as just the sublimox (and more about the sublimox than just source and price)