KWIK-E-MART (also known as KEM) farming guide.

Looking at what you posted about the cash and levels in your 4 towns then not for a while yet.
If you sell all the KEMs you can first you'd get 29 in one batch for a little less than the cost of a single RTT and that would get you 290,000 XP compared to 75,000 XP for the RTT - that's about 4 times the XP per $$, is before any bonus is applied and assumes you've got the collider running.
You need to wait until you've built up your town and bonus to earn fast before truck-farming

. . . or a simpler answer, when you don't look at the last six numbers in the cash counter any more and only check how many millions you have it's truck-farming time

First thing tomorrow after you collect everything from overnight write down what your cash is. Tap again as usual and 4 hours later write down what your cash has gone up to. Work out how much you made and spend no more than that building a bunch of KEMs. 4 hours later collect those and build the same amount again - you will earn extra because 1: you get 25% of your KEM spend back when you sell and 2: you get the 6, 8 12 hour buildings etc. income too going into your cash counter.
Those KEMs will help you level up and add sprinkles and the other money will go towards buying buildings and land etc, and eventually on trucks.
Until you get a bonus that you feel helps you earn fast enough spend those donuts only on Burns Boxes or bonus items that work out at 10 donuts for each 1% or less.
As you grow your bonus you'll find you earn more easily and can add more bonus faster
If you're really tempted by any of the premium offerings then for now consider paying real money for the sprinkles and ask yourself is it worth that much to me I'd pay for it -
If no I'd say stick to bonus stuff until you can clear more than 20 donuts a day ( 1,120 in 8 weeks will buy an event and mini-event worth of new content and feed the collider and add to your bonus )
Those are just thoughts, there is no right or wrong way to tap. It's your game to play as you like

Right now I just do KEM farming. Is it better to use decorations to get donuts? My multiplier is at 2500%. In this thread a person wrote that your game could crash if you buy a lot of decorations, is that true? I'd rather play it safe.

@Cousigames Only buy 10 or 20 rat traps or bloodmobiles at a time. The game can crash if you do too many at one time

I do 60 at a time, 5 rows of 12 RTTs will fit on a square the size of 9 road tiles each side and group select to sell in one.
The biggest trick for preventing crashing is don't do too many batches or farm if you've done a lot of other things too.
The game seems to keep a note of everything you tap / move / earn and if that list gets too big it goes pop. Just head to Friends Towns, quit the game and restart before you farm and if you farm a lot shut it down again after a while.
Doesn't help with the risk of Bart spoiling the party though !!